Abstract: The front suspension cylinder piston rod of a mine truck fractured during service. The fracture cause of piston rod were analyzed by means of macroscopic observation, chemical composition analysis, mechanical property test, fracture analysis and metallographic examination. The results show that the improper heat treatment process of the piston rod led to the induction quenching crack on the surface, fatigue crack was formatted from surface crack tip to interior when piston rod worked under hostile service condition. When the fatigue crack propagated to a certain extent, the load on the piston rod exceeded its bearing capacity and eventually led to its instantaneous fracture.