Abstract: Microstructures and mechanical properties of HR3C steel in as-received state and after 50 000 h service at high temperature were studied and compared. Main factors influencing the mechanical properties during service were analyzed. The results show that compared with the microstructure in as-received state, the microstructure of HR3C steel after service still consisted of austenite matrix and precipitates, but the amount and size of the precipitates increased. The precipitates on grain boundary and inside grain of austenite were mainly M₂₃C₆ carbide. After service, the tensile strength of HR3C steel changed little while the plasticity decreased; during bending, the steel had only a small amount of plastic deformation before brittle fracture. After service, the impact energy of HR3C steel was reduced and the impact fracture showed a typical intergranular fracture feature. The intergranular precipitation and growth of M₂₃C₆ carbide was the root cause of the decline of plasticity and toughness of HR3C steel.