Abstract: The Φ250 mm 3-roll, slight-stretch reducing mill of Tianjin Steel Tube Co. is imported from Mannesmann Demag, Germany. It employs a centralized differential drive system for all of its 14 roll stands, which means in case of rolling small-sized heavy-wall tubes, internal hexagonal shape is apt to occur during sizing. It is identified that improper speed setting of the mill is responsible for formation of the said internal hexagonal defect of the tube. Accordingly, corrective actions are proposed and implemented, including more effective groove design and optimizing the speeds of the main motor and the superimposed motor. As a result, the internal hexagonal defect of the final tube has been obviously reduced.