Optical microscopy (OM) and transmission electron microscopy (TEM) were used to investigate the effect of tempering temperature on the experimental extra-high carbon steels.It is found that tempering reaction can reduce austenite content and influence the stability of the austenite.As-normalized microstructure is a mixture of twinned martensite and retained austenite.Tempered at 250 ℃ for 2 h,lath martensite can occasionally be found nearby the diffusionally decomposed austenite arca.It also is found that tempering at 650 ℃ for 2 h,nanoparticles of carbides precipitate in the martensite and decomposed austenite.