Abstract: An aircraft undercarriage was found to have crack. The fracture mode and cause were analyzed by macroscopic and microscopic observation of fracture, metallographic examination, hardness testing and hydrogen content determination. The results show that the fracture mode is hydrogen embrittlement fracture. The plastic deformation caused in the process of repair resulted in local stress concentration, and no destressing process was carried out before electroplating, resulting in hudrogen embrittlement fracture. It is suggested that strict control of surface treatment process and destressing process should be carried out to reduce hydrogen embrittlement fracture.