Abstract: During precision machining of the quenched and tempered 40Cr steel load bearing connecting rod, the transverse cracking was found on its outer surface. The causes of cracking of the connecting rod were analyzed by macroscopic observation, chemical composition analysis, gas analysis, metallographic examination, and hardness testing. The results show that the reasons for the cracking of the connecting rod were due to improper heat treatment process, which resulted in the microstructure not being quenched thoroughly. Insufficient machining accuracy at the deformation of the inner groove of the part resulted in stress concentration, led to arc-shaped cracks extended to the surface. By adjusting the quenching process parameters, changing the cooling medium to water-based quenching liquid, optimizing the turning accuracy of the workpiece and changing the transition area to rounded corners, the qualification rate of the product could be improved.