Abstract: The influence of C contents on the microstructure evolution and stress rupture properties of K465 alloy were studied by investigating microstructure, DSC analysis and mechanical tests. The results show that the precipitation temperature and the quantity of the carbides are mainly affected by the content of C, which are both increased with the increase of C in the alloy. The transition temperature of eutectic and the quantity of eutectic decreases with the increase of C content. With the increase of C content, the quantity of carbides increases, while the quantity of eutectic and γ’ phase decreases. The fracture mechanism of the alloy during stress rupture deformation at 975 ℃/225 MPa is initiation of cracks from the grain boundary and the eutectic, and the cracks propagate along the grain boundary. As the increase of C content, the quantity of eutectic decreases, and the quantity of MC increases, which results in the improvement of the stress rupture time. However,the decrease of the quantity of γ’ phase with the C content increasing to 0.2 wt% would deteriorate the stress rupture life.