diff --git a/inc/class-wooviet-onepay-domestic.php b/inc/class-wooviet-onepay-domestic.php
index 8a6666b..a10bbf4 100644
--- a/inc/class-wooviet-onepay-domestic.php
+++ b/inc/class-wooviet-onepay-domestic.php
@@ -1,5 +1,4 @@
 <?php
-
 if ( ! defined( 'ABSPATH' ) ) {
 	exit;
 }
@@ -35,6 +34,7 @@ class WooViet_OnePay_Domestic extends WC_Payment_Gateway {
 		// Define user set variables.
 		$this->title          = $this->get_option( 'title' );
 		$this->description    = $this->get_option( 'description' );
+		$this->testmode       = 'yes' === $this->get_option( 'testmode', 'no' );
 		$this->merchant_id          = $this->get_option( 'merchant_id' );
 		$this->access_code          = $this->get_option( 'access_code' );
 		$this->secure_secret          = $this->get_option( 'secure_secret' );
@@ -65,22 +65,75 @@ class WooViet_OnePay_Domestic extends WC_Payment_Gateway {
 	 * @return string
 	 */
 	public function get_pay_url ( $order ) {
-		$access_key = $this->access_key;
-		$secret = $this->secret;               // require your secret key from 1pay
-		$return_url = $this->get_1pay_return_url();
-		$command = 'request_transaction';
-		$amount = $order->get_total();   // >10000
-		$order_id = $order->id;
-		$order_info = sprintf( 'The payment for the order number %1$s on the site: %2$s', $order->id, get_home_url());
+		$args = array(
+			'Title' =>  __('OnePay Payment Title', 'woo-viet'),
+			'vpc_Merchant' => $this->merchant_id,
+			'vpc_AccessCode' => $this->access_code,
+			'vpc_MerchTxnRef' => sprintf('%1$s_%2$s', $order->id, date ( 'YmdHis' )),
+			'vpc_OrderInfo' => substr(
+				sprintf('Order #%1$s - %2$s', $order->id, get_home_url()),
+				0,
+				32 ), // Limit 32 characters
+			'vpc_Amount' => $order->get_total() * 100, // Multiplying 100 is a requirement from OnePay
+			'vpc_ReturnURL' => $this->get_onepay_return_url(),
+			'vpc_Version' => '2',
+			'vpc_Command' => 'pay',
+			'vpc_Locale' => ( 'vi' == get_locale() ) ? 'vn' : 'en',
+			'vpc_Currency' => 'VND',
+			'vpc_TicketNo' => $_SERVER['REMOTE_ADDR'],
+		);
+
+		// Get the secure hash
+		$vpc_SecureHash = $this->create_vpc_SecureHash( $args );
+
+		// Add the secure hash to the args
+		$args['vpc_SecureHash'] = $vpc_SecureHash;
+		$http_args = http_build_query( $args, '', '&' );
+
+
+		if ( $this->testmode ) {
+			return 'https://mtf.onepay.vn/onecomm-pay/vpc.op?' . $http_args;
+		} else {
+			return 'https://onepay.vn/onecomm-pay/vpc.op?' . $http_args;
+		}
+
 
-		// @todo: review and see the file do.php in the demo files
 
 	}
-
-	// @todo review this function
-	/*
-	public function static get_return_url(){
+	// @todo: need to check this and WC_Payment_Gateway::get_return_url($order = NULL)
+	public function get_onepay_return_url(){
 		return WC()->api_request_url( __CLASS__ );
 	}
-	*/
+
+	/**
+	 * Get the PayPal request URL for an order.
+	 * @param  array $args
+	 * @return string
+	 */
+	public function create_vpc_SecureHash ( $args ){
+		$stringHashData = "";
+
+		// arrange array data a-z before make a hash
+		ksort ($args );
+
+		foreach($args as $key => $value) {
+
+			// tạo chuỗi đầu dữ liệu những tham số có dữ liệu
+			if (strlen($value) > 0) {
+				//sử dụng cả tên và giá trị tham số để mã hóa
+				if ((strlen($value) > 0) && ((substr($key, 0,4)=="vpc_") || (substr($key,0,5) =="user_"))) {
+					$stringHashData .= $key . "=" . $value . "&";
+				}
+			}
+		}
+		//xóa ký tự & ở thừa ở cuối chuỗi dữ liệu mã hóa*****************************
+		$stringHashData = rtrim($stringHashData, "&");
+
+		return strtoupper(hash_hmac('SHA256', $stringHashData, pack('H*', $this->secure_secret )));
+	}
+
+	public function process_return_url (){
+		// http://localhost/woo-viet/wc-api/WooViet_OnePay_Domestic/?vpc_AdditionData=686868&vpc_Amount=40000000&vpc_Command=pay&vpc_CurrencyCode=VND&vpc_Locale=en&vpc_MerchTxnRef=%23133_20170418024557&vpc_Merchant=ONEPAY&vpc_OrderInfo=%23133+-+http%3A%2F%2Flocalhost%2Fwoo-viet&vpc_TransactionNo=1576998&vpc_TxnResponseCode=0&vpc_Version=2&vpc_SecureHash=25C4443CFF95F6D5BCE4AD86DAA6756960688BD8868CBE898989125050504361
+	}
+
 }
diff --git a/inc/onepay/domestic-settings.php b/inc/onepay/domestic-settings.php
index baa53d4..aa77cd3 100644
--- a/inc/onepay/domestic-settings.php
+++ b/inc/onepay/domestic-settings.php
@@ -15,6 +15,13 @@ return array(
 		'label'   => __( 'OnePay Domestic Gateway (by Woo Viet)', 'woo-viet' ),
 		'default' => 'yes'
 	),
+	'testmode' => array(
+		'title'       => __( 'OnePay Sandbox', 'woocommerce' ),
+		'type'        => 'checkbox',
+		'label'       => __( 'Enable OnePay sandbox (testing)', 'woocommerce' ),
+		'default'     => 'no',
+		'description' => sprintf( __( 'OnePay sandbox can be used to test payments. See <a href="%s">the testing info</a>.', 'woocommerce' ), 'https://mtf.onepay.vn/developer/?page=modul_noidia' ),
+	),
 	'title' => array(
 		'title'       => __( 'Title', 'woo-viet' ),
 		'type'        => 'text',
@@ -27,7 +34,7 @@ return array(
 		'type'        => 'text',
 		'desc_tip'    => true,
 		'description' => __( 'This controls the description which the user sees during checkout.', 'woo-viet' ),
-		'default'     => __( 'With OnePay Bank, you can make payment by using any local Vietnam ATM card.', 'woo-viet' )
+		'default'     => __( 'With OnePay, you can make payment by using any local Vietnam ATM card.', 'woo-viet' )
 	),
 	'api_details' => array(
 		'title'       => __( 'API Credentials', 'woo-viet' ),
@@ -42,7 +49,7 @@ return array(
 		'desc_tip'    => true,
 		'placeholder' => __( 'Required. Provided by OnePay.', 'woo-viet' )
 	),
-	'access code' => array(
+	'access_code' => array(
 		'title'       => __( 'Access Code', 'woocommerce' ),
 		'type'        => 'text',
 		'description' => __( 'Get your Access Code from from OnePay.', 'woo-viet' ),