diff --git a/src/main/scala/com/fluency03/blockchain/core/Block.scala b/src/main/scala/com/fluency03/blockchain/core/Block.scala
index 0b612f2..a3e3ed9 100644
--- a/src/main/scala/com/fluency03/blockchain/core/Block.scala
+++ b/src/main/scala/com/fluency03/blockchain/core/Block.scala
@@ -21,8 +21,6 @@ case class Block(header: BlockHeader, transactions: Seq[Transaction], hash: Stri
   lazy val difficulty: Int = header.difficulty
   lazy val nonce: Int = header.nonce
 
-//  lazy val hash: String = header.hash
-
   def nextTrial(): Block = Block(header.nextTrial(), transactions)
 
   def addTransaction(tx: Transaction): Block =
@@ -34,10 +32,12 @@ case class Block(header: BlockHeader, transactions: Seq[Transaction], hash: Stri
   def removeTransaction(tx: Transaction): Block =
     Block(index, previousHash, data, timestamp, difficulty, nonce, transactions.filter(_ != tx))
 
-  def isValid: Boolean = isWithValidDifficulty(hash, difficulty) && hasValidMerkleHash
+  def hasValidHash: Boolean = hasValidHeaderHash && isWithValidDifficulty(hash, difficulty) && hasValidMerkleHash
 
   def hasValidMerkleHash: Boolean = merkleHash == Merkle.computeRoot(transactions)
 
+  def hasValidHeaderHash: Boolean = hash == header.hash
+
   def toJson: JValue = Extraction.decompose(this)
 
   override def toString: String = compact(render(toJson))
diff --git a/src/main/scala/com/fluency03/blockchain/core/Blockchain.scala b/src/main/scala/com/fluency03/blockchain/core/Blockchain.scala
index 5f5cb3c..d365c34 100644
--- a/src/main/scala/com/fluency03/blockchain/core/Blockchain.scala
+++ b/src/main/scala/com/fluency03/blockchain/core/Blockchain.scala
@@ -47,8 +47,8 @@ object Blockchain {
 
   def isValidChain(chain: Seq[Block]): Boolean = chain match {
     case Nil => true
-    case g +: Nil => g.previousHash == ZERO64 && g.index == 0 && g.isValid
-    case a +: b +: tail => canBeChained(a, b) && a.isValid && isValidChain(b +: tail)
+    case g +: Nil => g.previousHash == ZERO64 && g.index == 0 && g.hasValidHash
+    case a +: b +: tail => canBeChained(a, b) && a.hasValidHash && isValidChain(b +: tail)
   }
 
 
diff --git a/src/main/scala/com/fluency03/blockchain/core/Transaction.scala b/src/main/scala/com/fluency03/blockchain/core/Transaction.scala
index 1662d38..cd0e1d2 100644
--- a/src/main/scala/com/fluency03/blockchain/core/Transaction.scala
+++ b/src/main/scala/com/fluency03/blockchain/core/Transaction.scala
@@ -18,7 +18,6 @@ case class TxIn(previousOut: Outpoint, signature: String)
 case class TxOut(address: String, amount: Long)
 
 case class Transaction(txIns: Seq[TxIn], txOuts: Seq[TxOut], timestamp: Long, id: String) {
-//  lazy val id: String = hashOfTransaction(this)
 
   def addTxIn(in: TxIn): Transaction = Transaction(in +: txIns, txOuts, timestamp)
 
@@ -32,6 +31,8 @@ case class Transaction(txIns: Seq[TxIn], txOuts: Seq[TxOut], timestamp: Long, id
 
   def removeTxOut(txOut: TxOut): Transaction = Transaction(txIns, txOuts.filter(_ != txOut), timestamp)
 
+  def hasValidId: Boolean = id == hashOfTransaction(this)
+
   def toJson: JValue = Extraction.decompose(this)
 
   override def toString: String = compact(render(toJson))
diff --git a/src/test/scala/com/fluency03/blockchain/core/BlockTest.scala b/src/test/scala/com/fluency03/blockchain/core/BlockTest.scala
index cd6857a..b4f6384 100644
--- a/src/test/scala/com/fluency03/blockchain/core/BlockTest.scala
+++ b/src/test/scala/com/fluency03/blockchain/core/BlockTest.scala
@@ -30,7 +30,7 @@ class BlockTest extends FlatSpec with Matchers {
     genesis.timestamp shouldEqual expectedHeader.timestamp
     genesis.nonce shouldEqual expectedHeader.nonce
     genesis.hash shouldEqual expectedHeader.hash
-    genesis.isValid shouldEqual true
+    genesis.hasValidHash shouldEqual true
     genesis.toJson shouldEqual expectedBlockJson
     parse(genesis.toString) shouldEqual expectedBlockJson
 
@@ -68,7 +68,7 @@ class BlockTest extends FlatSpec with Matchers {
     genesisNextTrial.hash shouldEqual newHash
     genesisNextTrial.hash shouldEqual newExpectedHeader.hash
     genesisNextTrial.hasValidMerkleHash shouldEqual true
-    genesisNextTrial.isValid shouldEqual false
+    genesisNextTrial.hasValidHash shouldEqual false
     val headerJson = expectedHeader.toJson.transformField {
       case ("nonce", JInt(x)) => ("nonce", JInt(x+1))
     }
@@ -95,7 +95,7 @@ class BlockTest extends FlatSpec with Matchers {
     val newHash = hashOfBlockHeader(newBlock.header)
     newBlock.hash shouldEqual newHash
     newBlock.hasValidMerkleHash shouldEqual true
-    newBlock.isValid shouldEqual false
+    newBlock.hasValidHash shouldEqual false
     val headerJson = expectedHeader.toJson.transformField {
       case ("merkleHash", JString(_)) => ("merkleHash", newMerkleHash)
     }
@@ -127,7 +127,7 @@ class BlockTest extends FlatSpec with Matchers {
     val newHash = hashOfBlockHeader(newBlock.header)
     newBlock.hash shouldEqual newHash
     newBlock.hasValidMerkleHash shouldEqual true
-    newBlock.isValid shouldEqual false
+    newBlock.hasValidHash shouldEqual false
     val headerJson = expectedHeader.toJson.transformField {
       case ("merkleHash", JString(_)) => ("merkleHash", newMerkleHash)
     }
diff --git a/src/test/scala/com/fluency03/blockchain/core/BlockchainTest.scala b/src/test/scala/com/fluency03/blockchain/core/BlockchainTest.scala
index 5990065..21b0ddb 100644
--- a/src/test/scala/com/fluency03/blockchain/core/BlockchainTest.scala
+++ b/src/test/scala/com/fluency03/blockchain/core/BlockchainTest.scala
@@ -2,8 +2,10 @@ package com.fluency03.blockchain
 package core
 
 import com.fluency03.blockchain.core.Transaction.createCoinbaseTx
+import org.json4s.JsonDSL._
 import org.json4s.JValue
-import org.json4s.native.JsonMethods.parse
+import org.json4s.JsonAST.JArray
+import org.json4s.native.JsonMethods.{compact, parse, render}
 import org.scalatest.{FlatSpec, Matchers}
 
 import scala.io.Source
@@ -31,6 +33,11 @@ class BlockchainTest extends FlatSpec with Matchers  {
     blockchain.lastBlock().isEmpty shouldEqual false
     blockchain.lastBlock().get shouldEqual expectedGenesisBlock
     blockchain.isValid shouldEqual true
+    val json = ("difficulty" -> blockchain.difficulty) ~ ("chain" -> JArray(List(expectedBlockJson)))
+    blockchain.toJson shouldEqual json
+    parse(blockchain.toString) shouldEqual json
+
+    a[NoSuchElementException] should be thrownBy Blockchain(4, Seq.empty[Block]).isValid
   }
 
   "A new Blockchain with different difficulty" should "have all default values but the difficulty." in {