Abstract: The piston rod in hydraulic cylinder of a construction machinery fractured when it was used for only 200 h. The fracture causes of the piston rod were analyzed by means of macroscopic analysis, macroscopical examination, chemical composition analysis, metallographic examination and mechanical property test. The results show that the matrix of the piston rod was discontinuous due to the existence of large size inclusions. Under the action of dynamic tension-compression cyclic load, microcracks occurred at inclusions. In addition, the machining accuracy of screw thread was not good, and the crack expanded in the process of use, which eventually led to fatigue fracture of the piston rod.