High cycle fatigue properties of Alloy 718 for space use were investigated at temperature range from 77 to 873 K. The testing specimens were cut from welded joints as well as the base metals. The relationship between stress amplitude and fatigue life (S-N curve) had two steps for all the specimens at any temperatures, since the fracture mechanisms were different between low and high cycle regions. In low-cycle region, the origin of the fatigue fracture was the specimen surfaces, while in high-cycle region it was the facets inside or near the surface. And the fatigue strength depended on the austenitic grain size in high-cycle region. The facets, the origin of the fatigue fracture, corresponded to the grain size.