Abstract: The effect of microstructure on stress rupture property in fine cast IN 718 alloy having grain size of ASTM 3~5 has been investigated. Intergranular fracture as a main kind of creep fracture, and most of Laves phase, δ phase and carbides positioning in grain boundary in fine grain cast material have been found by analyzing the stress rupture fractograph and microstructure. The test results has exprienced that carbides and Laves phase in grain boundary will cause reducation of stress rupture properties.