Fatigue Strength properties of three types of press forming die steels for high tensile strength steel sheets were investigated. Cold work die steel, JIS SKD11 and high speed tool steel, JIS SKH51 were employed as the test material. In addition, newly developed cold work die steel, was less primary carbide than SKD11 was employed as the test material. Four point bending static and fatigue tests were conducted in air. From the experimental results of static bending test, it was found that the flexural strength was higher with increasing carbon content. From the results of fatigue test, Fatigue strength of newly developed cold work die steel was the highest value in three die steels. Fatigue limit of it showed 220 MPa higher than the comparative material steel, JIS SKD11 and 70 MPa higher than the high speed tool steel, JIS SKH51. From the results of fracture surface observation by SEM and EDX, fatigue strength of SKD11 and SKH51 were governed by stress intensity factor range ΔK_I. It was concluded that fatigue strength of newly developed cold work die steel was governed by intrusion and extrusion of the surface caused by surface roughness.
