diff --git a/pkg/core/blockchain.go b/pkg/core/blockchain.go
index 6a7b6526a..13266c65c 100644
--- a/pkg/core/blockchain.go
+++ b/pkg/core/blockchain.go
@@ -1181,7 +1181,7 @@ func (bc *Blockchain) ApplyPolicyToTxSet(txes []*transaction.Transaction) []*tra
 	)
 	blockSize = uint32(io.GetVarSize(new(block.Block)) + io.GetVarSize(len(txes)+1))
 	for i, tx := range txes {
-		blockSize += uint32(io.GetVarSize(tx))
+		blockSize += uint32(tx.Size())
 		sysFee += tx.SystemFee
 		if blockSize > maxBlockSize || sysFee > maxBlockSysFee {
 			txes = txes[:i]
@@ -1234,7 +1234,7 @@ func (bc *Blockchain) verifyAndPoolTx(t *transaction.Transaction, pool *mempool.
 		// Only one %w can be used.
 		return fmt.Errorf("%w: %v", ErrPolicy, err)
 	}
-	size := io.GetVarSize(t)
+	size := t.Size()
 	if size > transaction.MaxTransactionSize {
 		return fmt.Errorf("%w: (%d > MaxTransactionSize %d)", ErrTxTooBig, size, transaction.MaxTransactionSize)
 	}
diff --git a/pkg/core/blockchain_test.go b/pkg/core/blockchain_test.go
index bbf3c6351..7d2732ed1 100644
--- a/pkg/core/blockchain_test.go
+++ b/pkg/core/blockchain_test.go
@@ -524,8 +524,8 @@ func TestGetTransaction(t *testing.T) {
 		tx, height, err := bc.GetTransaction(block.Transactions[0].Hash())
 		require.Nil(t, err)
 		assert.Equal(t, block.Index, height)
+		assert.Equal(t, txSize, tx.Size())
 		assert.Equal(t, block.Transactions[0], tx)
-		assert.Equal(t, txSize, io.GetVarSize(tx))
 		assert.Equal(t, 1, io.GetVarSize(tx.Attributes))
 		assert.Equal(t, 1, io.GetVarSize(tx.Scripts))
 		assert.NoError(t, bc.persist())
diff --git a/pkg/core/transaction/transaction.go b/pkg/core/transaction/transaction.go
index 021708d67..bcf4d2af8 100644
--- a/pkg/core/transaction/transaction.go
+++ b/pkg/core/transaction/transaction.go
@@ -62,8 +62,8 @@ type Transaction struct {
 	// for correct signing/verification.
 	Network netmode.Magic
 
-	// feePerByte is the ratio of NetworkFee and tx size, used for calculating tx priority.
-	feePerByte int64
+	// size is transaction's serialized size.
+	size int
 
 	// Hash of the transaction (double SHA256).
 	hash util.Uint256
@@ -158,6 +158,7 @@ func (t *Transaction) DecodeBinary(br *io.BinReader) {
 	// to do it anymore.
 	if br.Err == nil {
 		br.Err = t.createHash()
+		_ = t.Size()
 	}
 }
 
@@ -252,18 +253,22 @@ func NewTransactionFromBytes(network netmode.Magic, b []byte) (*Transaction, err
 	if r.Err == nil {
 		return nil, errors.New("additional data after the transaction")
 	}
-	tx.feePerByte = tx.NetworkFee / int64(len(b))
+	tx.size = len(b)
 	return tx, nil
 }
 
 // FeePerByte returns NetworkFee of the transaction divided by
 // its size
 func (t *Transaction) FeePerByte() int64 {
-	if t.feePerByte != 0 {
-		return t.feePerByte
+	return t.NetworkFee / int64(t.Size())
+}
+
+// Size returns size of the serialized transaction.
+func (t *Transaction) Size() int {
+	if t.size == 0 {
+		t.size = io.GetVarSize(t)
 	}
-	t.feePerByte = t.NetworkFee / int64(io.GetVarSize(t))
-	return t.feePerByte
+	return t.size
 }
 
 // Sender returns the sender of the transaction which is always on the first place
@@ -296,7 +301,7 @@ type transactionJSON struct {
 func (t *Transaction) MarshalJSON() ([]byte, error) {
 	tx := transactionJSON{
 		TxID:            t.Hash(),
-		Size:            io.GetVarSize(t),
+		Size:            t.Size(),
 		Version:         t.Version,
 		Nonce:           t.Nonce,
 		Sender:          address.Uint160ToString(t.Sender()),
@@ -329,6 +334,9 @@ func (t *Transaction) UnmarshalJSON(data []byte) error {
 	if t.Hash() != tx.TxID {
 		return errors.New("txid doesn't match transaction hash")
 	}
+	if t.Size() != tx.Size {
+		return errors.New("'size' doesn't match transaction size")
+	}
 
 	return t.isValid()
 }
diff --git a/pkg/core/transaction/transaction_test.go b/pkg/core/transaction/transaction_test.go
index 216e7ca27..fd7705477 100644
--- a/pkg/core/transaction/transaction_test.go
+++ b/pkg/core/transaction/transaction_test.go
@@ -73,6 +73,7 @@ func TestNew(t *testing.T) {
 	assert.Equal(t, script, tx.Script)
 	// Update hash fields to match tx2 that is gonna autoupdate them on decode.
 	_ = tx.Hash()
+	_ = tx.Size()
 	testserdes.EncodeDecodeBinary(t, tx, &Transaction{Network: netmode.UnitTestNet})
 }
 
diff --git a/pkg/rpc/server/server_test.go b/pkg/rpc/server/server_test.go
index bf2ea7ed3..f7f5eb87c 100644
--- a/pkg/rpc/server/server_test.go
+++ b/pkg/rpc/server/server_test.go
@@ -811,6 +811,7 @@ func testRPCProtocol(t *testing.T, doRPCCall func(string, string, *testing.T) []
 	t.Run("getrawtransaction 2 arguments, verbose", func(t *testing.T) {
 		block, _ := chain.GetBlock(chain.GetHeaderHash(0))
 		TXHash := block.Transactions[0].Hash()
+		_ = block.Transactions[0].Size()
 		rpc := fmt.Sprintf(`{"jsonrpc": "2.0", "id": 1, "method": "getrawtransaction", "params": ["%s", 1]}"`, TXHash.StringLE())
 		body := doRPCCall(rpc, httpSrv.URL, t)
 		txOut := checkErrGetResult(t, body, false)