Hi I want to ask something. I'm usting in my connection parying devices by BluetoothSecurity.PairRequest(Device.DeviceAddress , pin);. After that my data transmision is encrypted? Or I have to do something more?

You should set BluetoothClient.Encrypt or BluetoothListener.Encrypt.  That will request that the Bluetooth stack ensure that the connection is encrypted.  Note that that property is only supported on the Microsoft Bluetooth stack (both desktop and WM).  (Enabling that sets the socket option, see http://msdn.microsoft.com/en-us/library/windows/desktop/aa362911.aspx )