The effects of warm working on microstructural, retained austenite characteristics and shear deformation properties of 0.2C-1.5Si-1.5Mn-1.0Cr-0.2Mo TRIP-aided martensitic (TM) steel for applications to automotive frame and forging parts were investigated. When warm working at 550 oC and post cooling at 1 oC/s was conducted to the TM steel, volume fractions of retained austenite and martensite-austenite constituent phase increased and mixture matrix of ultra fine granular bainitic ferrite and fine bainitic ferrite lath was obtained, whereas microstructure of TM steel warm worked at 750 oC exhibited granular bainitic ferrite matrix. These were caused by the dynamic recrystallization and the promotion of bainitic transformation of austenite due to the worm forging at 550 oC with the post cooling rate of 1 oC/s. Maximum shear stress decreased and total shear displacement increased with decreasing working temperature in TM steel. These were caused by the effective strain induced transformation of a large amount of retained austenite and the refined matrix structure.