Abstract: The power-assisted steering system of an automobile engine failed in running process. After dismantling and disposal, it was found that the idler gear of the engine cracked. In order to find the failure causes of the idler gear, the fracture analysis, chemical composition analysis, hardness testing, metallographic inspection and the force analysis were adopted. The results indicate that:the real force burdened by the iron wheel of the idler gear was higher than the designed max radical force, and the designed strength of the iron wheel was insufficient, which led to the failure and low-cycle fatigue cracking along the circular arc chamfering surface during use. Finally, the problem was solved effectively by the improvement of technological design.