Abstract: The cause of peeling of a rail thermite welding joint was analyzed by means of damage feature, metallographic structure, microhardness and chemical composition tests. The results show that no preheating during the arc welding repair for thermite welding joint leads to the formation of martensite in the heat affected zone of arc welding. Under the wheel-rail contact stress, microcracks are formed in the area near martensite, and eventually peeling occurs. The improper selection of electrode promotes the formation of peeling.