<p>Alloy 800H is considered a more advanced structural material than conventional stainless steels (SSs) and a potential candidate for next-generation nuclear reactors. As an improved version of alloy 800 with better creep resistance, the radiation damage behavior of alloy 800H remains insufficiently understood. To clarify the effect of compositional and microstructural differences on irradiation response, this study systematically compares both alloy 800 and 800H after proton irradiation to 2.5 dpa at 360&#xa0;°C. Dislocation loops and radiation-induced precipitates were observed in both alloys, whereas voids were additionally detected in alloy 800H. Radiation-induced Ni₃Si precipitates dominated the irradiated microstructure in both alloys, forming preferentially along dislocation lines in alloy 800 and within slip bands in alloy 800H. Alloy 800H exhibited a higher precipitate volume fraction, accompanied by void formation. These results suggest that the increased precipitation in alloy 800H was ineffective in suppressing swelling. The observed differences are attributed primarily to the slightly higher Si content in alloy 800H. The increased Si modifies stacking fault energy, alters dislocation structures, and promotes enhanced radiation-induced precipitation. During irradiation, the higher Si content drives additional Ni₃Si formation, reducing the availability of Ni and Si in solid solution, which are typical swelling resistant solutes, and leading to a shortened incubation dose for void nucleation. These findings highlight the critical role of minor compositional variations in governing radiation-induced precipitation and swelling behavior in alloy 800 and 800H.</p>