The “Quenching and Partitioning” (Q&P) process is a novel heat treatment designed for processing new generation advanced high strength steels (AHSS) with substantial ductility. In this study, evolution of complex microstructure for medium carbon steels during the Q&P process has been discussed in detail. Such steels have shown a complex multiphase microstructure consisted of fresh lath-martensite, fresh plate-martensite, transition carbide and/or cementite, isothermal martensite/lower bainite, and second twin-martensite after the one-step Q&P process (with the identical quenching and partitioning temperature). The morphology for the microstructure at room temperature after the two-step Q&P process (with different quenching and partitioning temperatures) demonstrated a little different. The formation of different microstructure for these two processes and their correlation with the mechanical properties are discussed.