Abstract: In order to solve the transverse wall thickness variation problem of thick-walled seamless steel tubes produced by hot rolling, a coupled thermo-mechanical finite element model was established to simulate the stretch reducing hot rolling process, and industrial trials were performed to verify the model. Based on simulation results, the authors analyzed temperature, strain and friction force distributions in the rolling process, studied the radial and circumferential flow laws of metal in single pass rolling, the transverse flow law in the whole rolling process and the formation of thickness variation, discussed the influence of temperature on metal flow behavior, and finally summarized the reasons of transverse wall thickness variation. The metal circumferential flow direction is from the roller top to roller gap after rolled by a single pass. Circumferential flow of metal near the ±30℃ position of roller groove angle is the most active, and circumferential flow of metal near the roller top and gap positions is much weaker. However, after the whole rolling process, metal flows from the roller top and gap position to the ±30℃ position of roller groove angle along circumference. This causes that the thickness of the ±30℃ position is bigger than the roller top and gap position. Temperature has huge influence on metal transverse flow behavior. Because of plastic work, metal temperature at the ±30℃ position of roller groove angle increases higher than other positions, which softens and lowers the resistance of this place. So the metal fluidity of the roller top and gap positions toward the ±30℃ position is strengthened, leading to tube cross section appearing a hexagonal bore.