Hot upsetting experiments were carried out in the temperature range 700-950℃ on steel powder preforms presintered at 700℃. The influence of hot upsetting temperature on the stress connected with phase transformation and restoration mechanism was specially studied by investigating hot deformation behavior and microstructure analysis. Following conclusions were drawn on the basis of the present study. -A deep minimum and maximum in stress were found in the temperature range 800-900℃ for almost all powder preforms. -The flow stress during hot upsetting was directly related to α-γ phase transformation. -The flow stress during hot upsetting was dependent on the amounts of high-strength austenite and low-strength ferrite for carbon added powder preforms. -Major restoration mechanism during hat deformation in the ferrite range is dynamic recovery.