Abstract: Briefed in the essay are the drive type of the 3-roll sizing (reducing) mill,and the current commonlyadapted roll gap phase position relationship of the adjacent stands. Analyzed are the main causes for the "inter nal hexagonal" defect or imperfection of the tube as hot-rolled with the said mill. Relevant proposals are put forward for the purpose of turning the "internal hexagonal" defect into an "internal dodecagonal" one,and thus improving the W.T. accuracy of the tube,including adding more frame stations of the mill,changing the arrangement type and drive direction of the stands,and reducing the phase angle of the adjacent stands.