The tempering resistance in double tempering of a new type of tool steels used in magnesium die casting machine quenched at 1080°C, 1120°C and 1150°C respectively was investigated. 4 hours of first tempering and double tempering at 600°C, 630°C, 660°C, 690°C and 720°C are applied. The minimum hardness value is 53.4HRC quenched at 1150°C and then tempered. The decomposition of tempered martensite lath was retarded after adding a number of alloying elements such as Cr, Co, W, Mo, V, according to observations by transmission electron microscopy. It also shows that the fine dispersion secondary carbide precipitates along the martensite grain surface and grain boundary. The use of X-ray diffraction has revealed that secondary carbides precipitates takes the form of M6C carbide and the retained austenite has transformed to martensite completely quenching at 1150°C. These microstructures would lead to the better tempering resistance.