Abstract: An NC38 drill pipe joint was found to have transversal cracks in the inner wall during machining outer circle and inner hole after quenching and tempering treatment. The causes of cracks in the inner wall of the drill pipe joint were analyzed by macro observation, chemical composition analysis, metallographic analysis and energy spectrum analysis. The results show that the steel ingot for the production of the drill pipe joint was not cut enough from the top and left residual shrinkage in the ingot, which caused defects in the round bar for the production of the joint and then resulted in cracking of the joint in the subsequent processing. Finally, relevant suggestions were put forward to avoid losses caused by such defects.