Abstract: The evolution of microstructure and hardness of low carbon plate steel during tempering at 500-700℃ was investigated. The steel was manufactured by RFC (Relaxation-Precipitation Controlling phase transformation) technique. A part of the plate was reheated to 930℃ and held for 1 h before quenched into water (RQ) as a comparison. RFC plate softened slowly with the increase of tempering temperature and even became harder between 600 and 650℃. Despite of a lower original hardness of RQ plate it softened faster during tempering. RPC steel and RQ steel are constituted by bainite and martensite microstructures before tempering. The microstructure of RFC steel did not change obviously except some bainitic laths coalescing were detected by means of SEM during tempering, while the boundary of bainitic laths of RQ steel disappeared quickly and finally formed polygonal ferrite forms accompanied a dramatic decrease in hardness. These results indicate that the thermo-stability of fine non-equilibrium microstructure is tightly related to their history.