Effects of microstructural change, tensile properties and impact property according to the change of austenitizing temperature and tempering temperature of AISI 51B20 steel were examined. Regardless of austenite grain size, lath martensite with needle and packet shapes was found at tempering temperature of $300^{circ}C{sim}400^{circ}C$. The needles of lath martensite changed to parallel packet at tempering temperature of $450^{circ}C{sim}600^{circ}C$. As tempering temperature increased, tensile strength, yield strength and hardness decreased, while elongation, ratio of reduction area and Charpy impact energy increased. Grain size increased when quenching temperature was $930^{circ}C$. Grain size had prominent effect on the mechanical properties of AISI 51B20 steel. Ratio of tensile strength/yield strength and yield strength autenitized at $880^{circ}C$ followed by tempering at $350^{circ}C{sim}450^{circ}C$ showed higher values than that of autenization at $930^{circ}C$ due to fine grain size.