The hot deformation behavior of 55SiMnMo steel was studied by hot isothermal compression tests at 950-1 100℃and strain rates of 0.01-10s-1 using a Gleeble 3500thermal simulation machine.Experimental results show that the peak stress increases with decreasing deformation temperature and increasing strain rate.When the strain rateε=0.01s-1,or whenε=0.1s-1 and the deformation temperature T≥1 000℃,the dynamic recrystallization(DRX)of 55SiMnMo steel occurs.The hot flow stress constitutive equation,peak strain equation,as well as critical stress and strain for DRX initiation are obtained based on the experimental data.A comparison between the theoretical and experimental results verifies the reliability of the flow stress equation. The hot deformation behavior of 55SiMnMo steel was studied by hot isothermal compression tests at 950-1 100 ° C and strain rates of 0.01-10s-1 using a Gleeble 3500thermal simulation machine. Experimental results show that the peak stress increases with decreasing deformation temperature and increasing the strain rate .When the strain rate ε = 0.01s-1, or whenε = 0.1s-1 and the deformation temperature T ≧ 1000 ° C, the dynamic recrystallization (DRX) of 55SiMnMo steel occurs. hot-flow stress constitutive equation, peak strain equation, as well as critical stress and strain for DRX initiation are obtained based on the experimental data. A comparison between the theoretical and experimental results verifies the reliability of the flow stress equation.