Abstract: During the bench test of a certain type of engine, cracking on the top surface of piston and wearing in the position of skirt were found. The fracture, chemical composition, microstructure, hardness and so on were analyzed, the results showed that the fatigue cracking appeared on the top surface of piston; the thermal fatigue cracks was caused by insufficient cooling in working and high temperature of the top surface of piston, the cracks extended under high pressure stress of alternating gas, and led to cracking of the piston at last.