The work aims to improve the steam oxidation resistance of Super304H by preparing an aluminum diffusion coating on it. The aluminum diffusion coating was deposited onto the Super304H steel through a conventional slurry aluminizing method. Its oxidation kinetics at 650 ℃ in pure steam was studied by discontinuous weighing method. The phase composition and microstructure of the aluminum diffusion coating and the oxide scale were characterized and analyzed by means of X-ray diffraction, scanning electron microscope and transmission electron microscope. During aluminizing, Al in the slurry was diffused into the substrate steel while Fe in the steel diffused outwards, forming an outer layer of FeAl close to the stoichiometric ratio followed by an intermediate layer of Al-depleted FeAl. Through the FeAl layers, Al was constantly diffused into the austenitic (γ) substrate, which induced the increased concentration of Al in the substrate. When the concentration of Al increased to a critical one, NiAl precipitate occurred locally. The nucleation and growth of NiAl precipitate further enhanced the depletion of Ni in its surrounding substrates. With both the depletion of Ni and the enrichment of Al intensifying in the γ substrate, the γ substrate became unstable and then gradually transformed into ferrite (α-Fe). Finally, an interdiffusion layer consisting of α-Fe dispersed with NiAl precipitates developed. Because of the combined action of the dissolved Cr in FeAl as well as the water steam molecule, a continuous and compact oxide scale of α-Al2O3 closely bonded to the substrate was formed on the aluminum diffusion coating in pure steam at 650 ℃, significantly slowing down the oxidation rate of the Super304H steel. Triple-layered aluminum diffusion coating on Super304H steel can be prepared by slurry aluminizing method, composed of a FeAl layer, an Al-depleted FeAl layer, and an interdiffusion layer of α-Fe dispersed with NiAl precipitates from outer to inner. The aluminum diffusion coating of such structure can decrease the oxidation rate of Super304H steel and therefore increase the steam oxidation resistance of the steel by forming the α-Al2O3 scale in the pure steam at 650 ℃.