Effect of aging heat treatment on the grain boundary microstructure and creep properties of a cast Ni-based superalloy was investigated.With increasing aging temperature from 750 to 1000 ℃,M23C6 carbides along the grain boundaries evolve from fine distributed block,continuous film into the coarse discrete block.Moreover,the M23C6 carbides are mainly enveloped within γ\'layers along grain boundaries during 1000 ℃ aging.Creep rupture lifetime and elongation at 760 ℃ and 645 MPa are improved with increasing the aging temperature.In particular,the creep rupture lifetime of the specimens aging at 1000 ℃ is one order of magnitude higher than that of the specimens aging at 750 ℃.The enhancement of ductility induced by the γ\'envelopes plays a significant role in the improvement of creep rupture lifetime.