Abstract: A large number of skin defect was found on inner surface of 37Mn5 steel tubes after wearing,and means such as macroscopic observation,chemical compositions analysis,scanning electron microscopy and energy spectrum analysis were used to analyze the formation causes of the skin defect.The results show that there were a lot of non-metallic inclusions in 37Mn5 round steel and the nitrogen content of it was rather high.So during the wearing,skin defect would appear at positions of inclusions and gas with the decrease of tube wall thickness.