Ni-based superalloys have been considered as the candidate materials of advanced ultra-super-critical (A-USC) steam turbine boilers because these materials have a superior creep strength, long term thermal stability, and resistance to oxidation. Inconel 740H is only available for the boiler material in the A-USC conditions among the Ni-based superalloys because it has been approved by section 1 of the ASME Boiler and Pressure Vessel Code. Meanwhile, most of the research has been focused on microstructural evolution and mechanical properties after long term thermal exposure at 750 ℃ without control of heat treatment condition. Therefore, in this research, heat treatments were designed for steam turbine boiler material to optimize microstructure stability and creep property. The materials were solution treated at 1,150 ℃ for 30 min followed by water-quenching, and sequentially aged at various condition. Each aging condition was suggested as Commercial (800 ℃/16 h/air-cooling, AC), Suggestion 1 (650 ℃/16 h + 800 ℃/16 h/AC) and Suggestion 2 (1,060 ℃/2 h/AC + 800 ℃/16 h/AC). The microstructure stability of heat treated alloys was investigated on thermal exposure at 750 ℃ for 5,000 h. After the thermal exposure, it was confirmed that grain size and γ′ size were similar in all heat treated alloys using optical microscopy and transmission electron microscopy (TEM). In addition, all alloys exhibited good microstructure stability showing slow coarsening of γ′ from 28 nm to 90 nm after the thermal exposure. From the tensile tests at 750 ℃, it was exhibited that all alloys performed approximately 590 MPa of yield strength and 32 % of elongation. After the thermal exposure (750 ℃/5,000 h), all alloys showed good mechanical properties exhibiting approximately 550 MPa of yield strength and 50 % of elongation. As a results of creep test at 750 ℃ under stress of 270 MPa, creep life of Suggestion 2 heat treated alloy (1,773 h) was superior to Commercial (1,546 h) and Suggestion 1 heat treated alloys (1,057 h). Since γ′ size was similar in the γ matrix of all heat treated alloys, characterization of grain boundary was conducted. Analysis using electron probe micro analyzer (EPMA) exhibited that Cr-rich M23C6 carbide and Nb, Ti-rich MC carbide were precipitated at the grain boundary. Volume fraction of MC carbide was estimated to be 33 % in Suggestion 2 heat treated alloy which is the highest value than Commercial (12 %) and Suggestion 1 heat treated alloys (10 %). It is conceivable that high fraction of MC carbide contributes to superior creep property because Suggestion 2 heat treated alloy exhibited the longest creep life. To investigate the effect of MC carbide at the grain boundary, TEM/energy dispersive spectroscopy analysis was conducted on the cross section of crept samples. It was observed that MC carbide was slightly decomposed with Cr partitioning at the interphase between MC carbide and γ matrix. It is expected that MC carbide has higher crack resistance than M23C6 carbide because external energies of temperature and stress of creep are consumed by decomposition-induced phase transformation (MC → M23C6). In addition, crept samples showed that the fractured area of M23C6 carbide was wider compared to that of MC carbide. Therefore, it was concluded that Suggestion 2 heat treatment, which leads to high fraction of MC carbide at the grain boundary, is the optimal condition for steam turbine boilers.