Abstract: It is commonly encountered with that defective inner spiral-line marks occur with seamless steel tubes being hot-rolled by either of the two typical piercing mills，i.e.，the barrel type rotary piercer and the cone type piercer.Causes for the said defect are investigated in terms of deformation zone and cold（thermal） rolling tools，and eventually identified.Accordingly，preventative measures are proposed，including adding one more cross-rolling stand，reducing the roll face angle of the roll outlet cone part，correctly selecting cold（thermal） rolling tolls，and preventing interference of any external element，etc.