public void writeCustomCharacteristic(int value) {
        if (mBluetoothAdapter == null || mBluetoothGatt == null) {
            Log.w(TAG, "BluetoothAdapter not initialized");
            return;
        }
        /*check if the service is available on the device*/
        BluetoothGattService mCustomService = mBluetoothGatt.getService(UUID.fromString("0000fff0-0000-1000-8000-00805f9b34fb"));
        if(mCustomService == null){
            Log.w(TAG, "Custom BLE Service not found");
            return;
        }
        /*get the read characteristic from the service*/
        BluetoothGattCharacteristic mWriteCharacteristic = mCustomService.getCharacteristic(UUID.fromString("0000fff1-0000-1000-8000-00805f9b34fb"));
        mWriteCharacteristic.setValue(value,android.bluetooth.BluetoothGattCharacteristic.FORMAT_UINT8,0);
        if(mBluetoothGatt.writeCharacteristic(mWriteCharacteristic) == false){
            Log.w(TAG, "Failed to write characteristic");
        }
    }

Below is my activity code:

public void onClickWrite(View v){
        if(mBluetoothLeService != null) {
            mBluetoothLeService.writeCustomCharacteristic(0x666666);
        }
    }