Abstract: Brittle fracture of the piston rod of construction machinery cylinder occurred in the early use. Through macroscopic analysis, chemical composition analysis, hardness test, fracture analysis and metallographic examination, the fracture causes of piston rod were analyzed. The results show that ferrite net and widmanstatten structure appeared in the piston rod due to improper heat treatment, which caused grain boundary embrittlement, and there were too much sulfides inclusions in the material. As a result, the ability of piston rod to bear alternating loads decreased and brittle fracture finally occurred.