Abstract: The flow stress of 20Mn2SiV micro-alloyed forging steel was investigated by MMS-300 thermo-mechanical simulator at the deformation temperature of 900~1100°C and the strain rates ranging from 0.01 to 10 s^-1. The relationship between Z parameter and dynamic recrystallization was discussed. A flow stress model of the steel was developed during hot deformation. The results showed that the occurrence of dynamic recrystallization can be judged by Z parameter. When lnZ is less than or equal to 32.76, the dynamic recrystallization takes place. The flow stress models were established, respectively, according to whether the dynamic recrystallization occurs or the strain is higher than the critical strain or not. The models show a good fitting precision.