Abstract: As a high-efficiency seamless steel tube rolling mill using continuously-cast billets as starting materials,the mandrel pipe mill is operated with a manufacturing process other than any conventional pipe-rolling process,which leads to different product defects compared with those as caused by the latter. Investigated are the causes responsible for the common defects of mandrel mill-rolled tubes. Based on the causes as identified,accurate product quality evaluation,defect cause analysis and control of defect occurrence at each manufacturing stage will be possible during different manufacturing stages such as operation,technical supervision and production management,resulting in better control of manufacturing steps to improve finalized product quality.