Due to the projected requirement for highly enhanced efficiency in power generation, the environment in advanced high temperature water-cooled nuclear systems becomes more rigorous compared to conventional fossil and nuclear power plants. The corrosion behavior of candidate alloys Inconel 617, 625, and 718 exposed to supercritical water was studied by a variety of analytical techniques. Grain boundary engineering (GBE) was performed on the alloy 617 to compare its corrosion behavior with the as-received samples. The results indicate that the GBE-treated alloy 617 samples showed the best response and the alloy 718 samples showed the worst corrosion resistance. Pitting observed on the alloy 718 samples was more serious than that on the alloy 625 samples. The effect of the alloying elements and the GBE treatment on the corrosion behavior is discussed.