This paper investigates the self-tempering of laser-hardened carbon tool steelsr by means of TEM. It has been found that there are a great deal of precipitation of boundaries in the maximun hardened region of T10A treated with laser hardening. The precipitated carbides, ranged from Inm to 10nm,have the Bhagaryatsku's orientation relation with a matrix of martensite. Based on the results, thermodynamic and dynamic conditions of the precipitation have also been analysed, Many observations show that the characteristic difference in micro-structures between T10A steels treated with laser hardening and those with traditional hardening indicates the existence of precipitation of carbides is a result of the self-tempering.