Thermal exposure experiments of Ni-based cast superalloy K403 at 950 °C for 5, 50 and 100 h were conducted. The effect of thermal exposure on the microstructures and room-temperature mechanical properties of the superalloys K403 was studied. The results show that M6C carbides segregate from intra-grains and grain boundaries, and γ' phases aggregate and grow up with blunted corners. With increasing of thermal exposure time, on the one hand, γ' phases of spherical or nearly spherical shapes are formed from blunted corners and parts of the γ' phase tend to coarsen due to the connection-oriented effect. On the other hand, the offset yielding stress and the tensile strength decrease, while the plasticity improves obviously, resulting from the aggregation and coarsening of the γ' strengthening phase. The room-temperature tensile fracture surface is characterized by the dendritic structure fracture for K403 alloys before and after the thermal exposure. Whereas the intercrystalline fracture feature, and the shallow and small dimples appear on the tensile section of the thermal-exposed alloys. And the amount of dimples increases as the exposure time increases.