The effect of Al addition on the corrosion behavior of high entropy alloys AlxCoCrFeNi (x = 0, 0.1, and 1.0) in supercritical water was investigated. The weight gain and oxide film thickness reduced with the increase of Al content. A duplex-structure oxide film, consisting of FeCr2O4 outer and Cr2O3 inner layer, was developed on CoCrFeNi alloy. A triple-layered oxide film consisting of FeCr2O4 outer, Cr2O3 middle, and Al2O3 inner layer was developed on Al0.1CoCrFeNi and AlCoCrFeNi alloys. The improvement of corrosion resistance by adding Al was attributed to the high Al concentration that accelerated the formation of Al2O3.