Stress relaxation behaviour of GH4145 alloy at 566 oC was investigated. The results showed that the alloy has good resistance to stress relaxation. During the stress relaxation test, although the average grain size of the alloy obviously increases, the coarsening rate of secondary γ′ phase is very slow. The increase of the volume fraction of cuboidal secondary γ′ particles with larger size and the precipitation of spherical tertiary γ′ particles with smaller size make the hardness of the alloy increase.