The object of the present work is to investigate the effect of cooling rate on microstructure and mechanical properties of AISI 1020, AISI 1040 and AISI 1060 steels. The samples were heated and treated at 1250 degrees K for 4 h and subsequently were cooled by three different methods. For this purpose, the microhardness and microstructure of these steels after heat treatment were examined by optical microscopy and hardness tests, respectively. Experimental results have shown that the microstructure of these steels can be changed and significantly improved by varying the cooling rate. Thus, heat treatment (heating and cooling) is used to obtain desired properties of steels such as improving the toughness, ductility or removing the residual stresses, etc.