A grain boundary precipitation treatment was studied for the purpose of improving the high-temper-ature low-cycle fatigue strength of a Ni-23% Cr- 18% W alloy, SSS113M, which had been developed as an intermediate heat exchanger material of very high temperature reactors and evaluated as the best alloy in the national R&D program of nuclear steelmaking in Japan. A conventional standard solution treatment of 1300°C × 1 h water quenched does not cause any grain boundary precipitation in SSS113M, but an additional heat treatment of 1250°C × 1 h causes discontinuous grain boundary precipitation of the alpha-tungsten phase. This grain boundary precipitation treatment results in two- to fivefold increases of low-cycle fatigue lives at 800°C as well as slight increases of the creep and stress rupture strength at 1000°C.