Abstract: Hot deformation behaviors of ultra-low carbon microalloyed steel were investigated in the temperature of 950-1150 ℃ and strain rate of 0.001-1 s^-1on Gleeble-1500D thermal simulation tester. The effects of deformation temperature and strain rate on the peak stress of alloy steel were studied. The results show that the peak stress decreases with deformation temperature increasing and increases with strain rate increasing. The Arrhenius constitutive equation based on peak stress of alloy steel is established, and its high prediction accuracy is proved by mathematical method.