Ni-base superalloys are used under sever conditions like a high temperature or sever corrosive environment because of their superior properties on corrosion, creep, static strength at high temperatures and so on. Therefore, many studies on creep and fatigue properties of the alloy have been carried out, e.g. Brown and Hicks [
]. Recently, it is very important to know the fatigue properties in long life region, because machines and structures are used for long-term from the point of view of environmental and economical demands. However, most of the studies on fatigue were very limited within the short life up to 10
cycles. It is reported that fracture occurs from an internal inclusion in long life region in many high strength steels and surface treated steels, while surface fracture occurs in short life region, e.g. Murakami
]. Moreover, the fatigue properties may be influenced by microstructure. However, the mechanism and the evaluation method for fatigue life of internal fracture are not clarified.