A motor vehicle security system including a receiver unit, an alarm unit, and a transmitter unit respectively mounted in a motor vehicle and connected to a car battery of the motor vehicle, the alarm unit and the transmitter unit are respectively connected to the receiver unit, the receiver unit being controlled by a remote controller to provide a check code to the alarm unit and the transmitter unit, the receiver unit been stopped from providing the check code when a door of the motor vehicle is opened by force, the alarm unit and the transmitter unit having a respective self-provided battery power supply and being triggered to produce an audio alarm signal and a radio alarm message respectively when they receive no check code from the receiver unit, the radio alarm message being receivable to car radios.