An automobile clutch system is operating under high temperature and repeating stress in clutch housing, whenever push clutch pedal. A diaphragm spring plays an important role at engage or disengage with release bearing. The material of spring, 50CrV4 is required as sufficient fatigue strength and tenacity, is very special spring steel. A diaphragm spring has a various concentrated stress, crack grow up and broken. It is very difficult to find the optimal heat treatment process. In this paper, to obtain the optimal heat treatment condition, the specimen is made of different tempering temperature and tested structural of surface, hardness, tensile strength. By fatigue crack propagation test, it is found the relation between fatigue crack growth rate and stress intensity factor range. On the broken surface, it is well distinguished with tempering temperature.