A protection circuit for power of a car is located between an output terminal of a car power and a car's electronic device input terminal. The protection circuit includes a transient voltage suppressing diode, a diode, and a filtering circuit. The two terminals of the transient voltage suppressing diode are connected with the output terminal of car power. The diode is connected with the transient voltage suppressing diode and the output terminal of car power. The filtering circuit is connected with the diode and the car's electronic device input terminal. Thereby, the transient voltage suppressing diode of the present invention clamps the ripple at a safe voltage to assure the life of the car's electronic device. Furthermore, the transient voltage suppressing diode and the filtering circuit can prevent the car's electronic device from being interfered and damaged due to the ESD. The reliability of the electronic device is enhanced.