In this paper the emphasis is placed upon fracture toughness and fracture surface morphology of cold forging die steels with Rockwell C scale hardness number of 52 to 68. Three point bending fracture toughness tests were conducted by use of Charpy impact specimens with fatigue crack for ten kinds of cold forging die steel. The higher the hardness the lower the fracture toughness is. The fracture toughness and crack initiation energy are well correlated to hardness. The Charpy impact energy of 5mmU notched specimen, 2.5mm saw cut specimen and 2.5mm saw cut specimen with fatigue crack is also well correlated to hardness. Well defined stretched zone was observed between fatigue and unstable fracture surface of SKD61 steel with Rockwell C scale hardness number with 52, ill defined stretched zone was observed on fracture surfaces of harder steels such as cemented carbide. The relation between stretched zone width and fracture toughness can be expressed as SZW(μm)=3.28(Kc/σ_y)^<1.24> for structural materials including cold forging die steels in the range of SZW between 150 and 0.5μm.
