Abstract: The Fe-0.05C-1.3Mn-0.35Ni-0.3Si-0.25Mo ultra-low carbon steel was compression deformed at 850℃, and then treated by 4 different ways of high-temperature staying+direct cooling, high-temperature staying+isothermal treating, direct isothermal treating and direct cooling, respectively. The phase composition, microstructure and hardness of the tested steel after different treatments were studied, and the influence of high-temperature staying on the intermediate-temperature phase transformation characteristic of the steel was analyzed. The results show that comparing to the direct cooling treatment, the intermediate-temperature phase transformation of the tested steel occurred and continued more easily due to the high-temperature staying and the hardness of the steel was relatively high. The high-temperature staying treatment transformed the dislocation entanglement produced during the deformation into the dislocation cell structure, which was benefiting for the formation of sheaf-like bainite, resulting in the increase of hardness of the tested steel.