Using Oyane ductile fracture criterion, a 2-D coupled thermo-mechanical simulation of center crack occurrence in round billet during 2-roll rotary rolling process is presented with the aid of FE code MSC.Superform. Based on an existing flat-die-model, an improved FE model is advanced, in which not only the influence of roll feed angle and entry cone angle but also the diameter of the piercing roll on the process are taken into account. By adopting the parameters of the Diescher piercer in 140mm Mandrel Mill of Bao Steel in China, the distribution and development of strain/stress in tube rounds are analyzed, and the eigenvalue of ductile fracture as well. The critical percentage of diameter reduction is obtained from the simulation. The result shows a good agreement with the experimental value, and therefore is of widely guiding significance to practical process for rationally formulating deformation parameters of steel tube piercing.