Susceptibility of hydrogen embrittlement of a super grade AISI 420 tool steel was studied. Tensile samples were cathodically charged to different hydrogen level. Hydrogen induced mechanical property degradation was measured by tensile tests at a low strain rate. Fractography of broken surfaces was observed using SEM. Relationship between hydrogen content and tensile strength and elongation were studied. Critical hydrogen contents were obtained for different heat treatment states. It was found that for annealed materials could stand for a 3.5ppm hydrogen for keeping 80% of original ductility, and the effect of hydrogen on strength was unobvious. However, for material quenched and tempered at 250°C, only 0.3ppm hydrogen could lead the ductility drop to 80% of original. The material quenched and tempered at 500°C was more sensitive on hydrogen, less than 1ppm hydrogen could lead the strength drop to 80% of original.