Abstract: The drill pipe was flattened many times during the drilling operation in a well. The causes of drill pipe flattening were analyzed by means of macro observation, chemical composition analysis, mechanical property test and metallographic examination. The results show that the uneven meshing contact between the drill pipe body and the slip caused the uneven stress distribution, when the local contact stress exceeded the yield strength of the drill pipe body material, the drill pipe body was plastically deformed and flattened. The serious wear of the slip teeth plate was the main reason for the uneven stress distribution.