diff --git a/chain/mining.go b/chain/mining.go
index 312457dfa..03d6f5671 100644
--- a/chain/mining.go
+++ b/chain/mining.go
@@ -33,6 +33,26 @@ func NewBuiltinMining(account *vault.Account, txnCollector *TxnCollector) *Built
 	}
 }
 
+func isBlockFull(txnCount uint32, txnSize uint32) bool {
+	if config.Parameters.NumTxnPerBlock > 0 && txnCount > config.Parameters.NumTxnPerBlock {
+		return true
+	}
+	if config.MaxBlockSize > 0 && txnSize > config.MaxBlockSize {
+		return true
+	}
+	return false
+}
+
+func isLowFeeTxnFull(txnCount uint32, txnSize uint32) bool {
+	if config.Parameters.NumLowFeeTxnPerBlock > 0 && txnCount > config.Parameters.NumLowFeeTxnPerBlock {
+		return true
+	}
+	if config.Parameters.LowFeeTxnSizePerBlock > 0 && txnSize > config.Parameters.LowFeeTxnSizePerBlock {
+		return true
+	}
+	return false
+}
+
 func (bm *BuiltinMining) BuildBlock(ctx context.Context, height uint32, chordID []byte, winnerHash common.Uint256, winnerType pb.WinnerType) (*block.Block, error) {
 	var txnList []*transaction.Transaction
 	var txnHashList []common.Uint256
@@ -44,9 +64,9 @@ func (bm *BuiltinMining) BuildBlock(ctx context.Context, height uint32, chordID
 	coinbase := bm.CreateCoinbaseTransaction(GetRewardByHeight(height) + donationAmount)
 	txnList = append(txnList, coinbase)
 	txnHashList = append(txnHashList, coinbase.Hash())
-	totalTxsSize := coinbase.GetSize()
-	txCount := 1
-	lowFeeTxCount := 0
+	totalTxSize := coinbase.GetSize()
+	totalTxCount := uint32(1)
+	var lowFeeTxCount, lowFeeTxSize uint32
 
 	if winnerType == pb.TXN_SIGNER {
 		if _, err = DefaultLedger.Store.GetTransaction(winnerHash); err != nil {
@@ -57,8 +77,8 @@ func (bm *BuiltinMining) BuildBlock(ctx context.Context, height uint32, chordID
 			}
 			txnList = append(txnList, miningSigChainTxn)
 			txnHashList = append(txnHashList, miningSigChainTxn.Hash())
-			totalTxsSize = totalTxsSize + miningSigChainTxn.GetSize()
-			txCount++
+			totalTxSize += miningSigChainTxn.GetSize()
+			totalTxCount++
 		}
 	}
 
@@ -76,22 +96,17 @@ func (bm *BuiltinMining) BuildBlock(ctx context.Context, height uint32, chordID
 		default:
 		}
 
-		if txCount >= int(config.Parameters.NumTxnPerBlock) {
-			break
-		}
-
 		txn := txnCollection.Peek()
 		if txn == nil {
 			break
 		}
 
-		if txn.UnsignedTx.Fee < int64(config.Parameters.MinTxnFee) && uint32(lowFeeTxCount) >= config.Parameters.NumLowFeeTxnPerBlock {
-			log.Warning("transaction fee is too low")
+		if isBlockFull(totalTxCount+1, totalTxSize+txn.GetSize()) {
 			break
 		}
 
-		totalTxsSize = totalTxsSize + txn.GetSize()
-		if totalTxsSize > config.MaxBlockSize {
+		if txn.UnsignedTx.Fee < int64(config.Parameters.MinTxnFee) && isLowFeeTxnFull(lowFeeTxCount+1, lowFeeTxSize+txn.GetSize()) {
+			log.Info("Low fee transaction full in block")
 			break
 		}
 
@@ -121,9 +136,11 @@ func (bm *BuiltinMining) BuildBlock(ctx context.Context, height uint32, chordID
 		}
 
 		bvs.Commit()
-		txCount++
+		totalTxCount++
+		totalTxSize += txn.GetSize()
 		if txn.UnsignedTx.Fee < int64(config.Parameters.MinTxnFee) {
 			lowFeeTxCount++
+			lowFeeTxSize += txn.GetSize()
 		}
 	}
 
diff --git a/chain/pool/txpool.go b/chain/pool/txpool.go
index e0adedcfd..e0b71353a 100644
--- a/chain/pool/txpool.go
+++ b/chain/pool/txpool.go
@@ -285,7 +285,7 @@ func (tp *TxnPool) processTx(txn *transaction.Transaction) error {
 		tp.blockValidationState.Commit()
 	default:
 		if oldTxn, err := list.Get(txn.UnsignedTx.Nonce); err == nil {
-			log.Warning("replace old tx")
+			log.Debug("replace old tx")
 			tp.blockValidationState.Lock()
 			defer tp.blockValidationState.Unlock()
 			if err := tp.CleanBlockValidationState([]*transaction.Transaction{oldTxn}); err != nil {
diff --git a/util/config/config.go b/util/config/config.go
index 9b3b69ecd..723b7322b 100644
--- a/util/config/config.go
+++ b/util/config/config.go
@@ -138,7 +138,8 @@ var (
 		WalletFile:                "wallet.json",
 		MaxGetIDSeeds:             3,
 		DBFilesCacheCapacity:      100,
-		NumLowFeeTxnPerBlock:      4,
+		NumLowFeeTxnPerBlock:      0,
+		LowFeeTxnSizePerBlock:     4096,
 		MinTxnFee:                 10000000,
 	}
 )
@@ -161,6 +162,7 @@ type Configuration struct {
 	CAPath                    string        `json:"CAPath"`
 	GenesisBlockProposer      string        `json:"GenesisBlockProposer"`
 	NumLowFeeTxnPerBlock      uint32        `json:"NumLowFeeTxnPerBlock"`
+	LowFeeTxnSizePerBlock     uint32        `json:"LowFeeTxnSizePerBlock"` // in bytes
 	MinTxnFee                 int64         `json:"MinTxnFee"`
 	RegisterIDRegFee          int64         `json:"RegisterIDRegFee"`
 	RegisterIDTxnFee          int64         `json:"RegisterIDTxnFee"`
@@ -250,6 +252,7 @@ func Init() error {
 		if Parameters.SyncBatchWindowSize == 0 {
 			Parameters.SyncBatchWindowSize = defaultSyncBatchWindowSize
 		}
+		log.Printf("Set SyncBatchWindowSize to %vMB", Parameters.SyncBatchWindowSize)
 	}
 
 	err := Parameters.verify()