diff --git a/src/main/java/conflux/web3j/Account.java b/src/main/java/conflux/web3j/Account.java
index 20c92ee..860ec5c 100644
--- a/src/main/java/conflux/web3j/Account.java
+++ b/src/main/java/conflux/web3j/Account.java
@@ -8,31 +8,47 @@
 import org.web3j.abi.FunctionEncoder;
 import org.web3j.abi.datatypes.Function;
 import org.web3j.abi.datatypes.Type;
+import org.web3j.crypto.Credentials;
 import org.web3j.crypto.RawTransaction;
 
 public class Account {
 	
 	private Cfx cfx;
 	private AccountManager am;
+	private Credentials credentials;
 	private String address;
 	
 	private BigInteger gasPrice = CfxUnit.DEFAULT_GAS_PRICE;
 	private BigInteger nonce;
 	
-	public Account(Cfx cfx, AccountManager am, String address, String password) throws Exception {
-		this(cfx, am, address, password, Duration.ZERO);
+	private Account(Cfx cfx, String address) {
+		this.cfx = cfx;
+		this.address = address;
+		this.nonce = cfx.getTransactionCount(address).sendAndGet();
 	}
 	
-	public Account(Cfx cfx, AccountManager am, String address, String password, Duration unlockTimeout) throws Exception {
+	public static Account unlock(Cfx cfx, AccountManager am, String address, String password) throws Exception {
+		return unlock(cfx, am, address, password, Duration.ZERO);
+	}
+	
+	public static Account unlock(Cfx cfx, AccountManager am, String address, String password, Duration unlockTimeout) throws Exception {
 		if (!am.unlock(address, password, unlockTimeout)) {
 			throw new Exception("account not found in keystore");
 		}
 		
-		this.cfx = cfx;
-		this.am = am;
-		this.address = address;
+		Account account = new Account(cfx, address);
+		account.am = am;
 		
-		this.nonce = cfx.getTransactionCount(address).sendAndGet();
+		return account;
+	}
+	
+	public static Account create(Cfx cfx, String privateKey) {
+		Credentials credentials = Credentials.create(privateKey);
+		
+		Account account = new Account(cfx, credentials.getAddress());
+		account.credentials = credentials;
+		
+		return account;
 	}
 	
 	public BigInteger getNonce() {
@@ -49,7 +65,9 @@ public Account withGasPrice(BigInteger gasPrice) {
 	}
 	
 	public String send(RawTransaction tx) throws Exception {
-		String signedTx = this.am.signTransaction(tx, this.address);
+		String signedTx = this.credentials == null
+				? this.am.signTransaction(tx, this.address)
+				: AccountManager.signTransaction(tx, this.credentials);
 		String txHash = this.cfx.sendRawTransaction(signedTx).sendAndGet();
 		
 		if (txHash == null || txHash.isEmpty()) {
diff --git a/src/main/java/conflux/web3j/AccountManager.java b/src/main/java/conflux/web3j/AccountManager.java
index 66112e4..1930943 100644
--- a/src/main/java/conflux/web3j/AccountManager.java
+++ b/src/main/java/conflux/web3j/AccountManager.java
@@ -247,10 +247,10 @@ public boolean unlock(String address, String password, Duration... timeout) thro
 		
 		UnlockedItem item;
 		
-		if (timeout == null || timeout.length == 0 || timeout[0].isNegative() || timeout[0].isZero()) {
-			item = new UnlockedItem(credentials, Optional.empty());
-		} else {
+		if (timeout != null && timeout.length > 0 && timeout[0] != null && timeout[0].compareTo(Duration.ZERO) > 0) {
 			item = new UnlockedItem(credentials, Optional.of(timeout[0]));
+		} else {
+			item = new UnlockedItem(credentials, Optional.empty());
 		}
 		
 		this.unlocked.put(address, item);
@@ -278,7 +278,10 @@ public boolean lock(String address) {
 	 */
 	public String signTransaction(RawTransaction tx, String address, String... password) throws Exception {
 		Credentials credentials = this.getCredentials(address, password);
-		
+		return signTransaction(tx, credentials);
+	}
+	
+	public static String signTransaction(RawTransaction tx, Credentials credentials) {
 		byte[] encodedTx = TransactionEncoder.encode(tx);
 		Sign.SignatureData signature = Sign.signMessage(encodedTx, credentials.getEcKeyPair());
 		List<RlpType> fields = TransactionEncoder.asRlpValues(tx, signature);
@@ -292,8 +295,9 @@ public String signTransaction(RawTransaction tx, String address, String... passw
 	}
 	
 	private Credentials getCredentials(String address, String... password) throws IOException, CipherException {
-		if (password == null || password.length == 0) {
-			UnlockedItem item = this.unlocked.get(address);
+		UnlockedItem item = this.unlocked.get(address);
+		
+		if (password == null || password.length == 0 || password[0] == null || password[0].isEmpty()) {	
 			if (item == null) {
 				throw new IllegalArgumentException("password not specified for locked account");
 			}
@@ -305,6 +309,14 @@ private Credentials getCredentials(String address, String... password) throws IO
 			
 			return item.getCredentials();
 		} else {
+			if (item != null) {
+				if (!item.expired()) {
+					return item.getCredentials();
+				}
+				
+				this.unlocked.remove(address);
+			}
+			
 			List<Path> files = Files.list(Paths.get(this.dir))
 					.filter(path -> this.parseAddressFromFilename(path.getFileName().toString()).equalsIgnoreCase(address))
 					.collect(Collectors.toList());
@@ -319,7 +331,10 @@ private Credentials getCredentials(String address, String... password) throws IO
 	
 	public String signMessage(byte[] message, boolean needToHash, String address, String... password) throws Exception {
 		Credentials credentials = this.getCredentials(address, password);
-		
+		return signMessage(message, needToHash, credentials);
+	}
+	
+	public static String signMessage(byte[] message, boolean needToHash, Credentials credentials) {
 		Sign.SignatureData data = Sign.signMessage(message, credentials.getEcKeyPair(), needToHash);
 		
 		byte[] rsv = new byte[data.getR().length + data.getS().length + data.getV().length];