Flow stress usually does not decrease with the increase of the deformation temperature of steel deformed at high temperature in two-phase region, and the minimum value of the flow stress will be found in the two-phase region. In order to find out the effect of chemical compositions on the flow stress behavior in the two-phase region, the deformation behavior of studied steels near two-phase region is researched by thermal simulation test. Meanwhile, the microstructures after deformation and the change of flow stress near the two-phases region are analyzed. The ferrite phase transformation start temperatures and the decrease of flow stress in two-phases region are obtained. The influence of main alloying elements such as C, Si and Mn on the ferrite transformation start temperature Ar₃ and the decrease of flow stress are obtained by regression fitting.