Abstract: The microstructure evolution during multi-pass warm deformation at 550℃ and subsequent annealing process of lath martensite was investigated and compared with that of SPD (severe plastic deformation) cold-rolled lath martensite during annealing at 550℃. In comparison with tempered martensite at the same temperature, the effects of warm and cold deformation on ferrite grain size and cementite precipitation were discussed. The results showed that both warm and cold deformation significantly promoted the precipitation of cementite as well as recovery and recrystallization of ferrite. The grain size of recrystallized ferrite after warm deformation was 0.5μm, meanwhile rod-type cementite particles changed to spherical-type, the size of cementite grew along with the increase of deformation reduction. Then ferrite grains grew up to 1μm when subsequently annealed at 550℃ for 60 min, and Ostwald ripening occurred during annealing process. In cold-rolled specimens which annealed at 550℃for 30 min, 0.3-0.4μm ferrite grains and dispersed cementite particles with the size below 150 nm formed. The mean sizes of ferrite grains and cementite particles grew up to 1.9 μm and 160 nm during subsequent annealing.