Abstract: The fracture reason of 65Mn locking plate in assembly process was analyzed by means of macroscopic examination, chemical composition analysis, hardness testing, metallographic examination, fracture analysis and so on. The results showed that the brittle fracture appeared in locking plate under assembly force due to temper brittleness caused by improper tempering processing in heat treatment. By adjusting the tempering process parameters, the temper brittleness was eliminated. Now locking plate in assembly process is normal, the product quality is ensured.