A combined process of hot deformation with different deformation temperatures plus two step quenching and partitioning (Q&P) treatment was applied to low carbon alloyed steel. The effect of stress (30% plastic deformation) on the start temperature of martensite transformation (Ms), volume fraction of retained austenite and mechanical properties was analyzed. It found that comparing with specimen treated by conventional two-step Q&P process, the microstructure of steel treated by combined process was finer and finer with the decreasing hot deformation temperature, and the typical curved micromorphology of martensite exists. Moreover, the Ms of specimen treated by combined process is increased with the increasing of deformation temperature. The effect of stress on the Ms can be attributed to the effect of stress on the grain boundaries of austenitic parent phase, where a large amount of dislocation induced by the stress is prior to occur so as to promote formation of martensite. However, the stability of untransformed austenite was improved by the plastic deformation when matensite transformed so as to get the more retained austenite (the highest volume fraction of retained austenite obtained by combined process of hot deformation at 750 ℃ is 17.2%). Moreover, the mechanical properties were improved by the combined process, namely, the highest hardness of specimen were obtained when hot deformation at 650 ℃ and the highest plasticity were obtained when hot deformed at 750 ℃.