When 2.25Cr-1Mo steel was hot-dip aluminized, a coating consisting of (Al-rich topcoat)/(Al13Fe4 layer)/(Al5Fe2 layer) formed. With increment of oxidation temperature, the transform of Al→Al₁₃Fe₄→Al₅Fe₂→AlFe→AlFe₃→α-Fe(Al)→α-Fe ℃curred through interdiffusion between the coating and the substrate as follows. The oxidation at 700 ℃ changed the coating to (α-Al2O3 scale)/(α-Al₂O₃, Al₁₃Fe₄, Al₅Fe₂)-mixed layer)/(Al₅Fe₂ layer)/(AlFe layer)/(AlFe₃ layer)/(α-Fe(Al) layer). The oxidation at 800 ℃ changed the coating to (α-Al2O3 scale)/((α-Al2O3, Al5Fe2)-mixed layer)/(Al5Fe2 layer)/(AlFe layer)/(AlFe₃ layer)/(α-Fe(Al) layer). The oxidation at 900 ℃ changed the coating to (α-Al₂O₃ scale)/((α-Al₂O₃, AlFe)-mixed layer)/(AlFe layer)/(AlFe₃ layer)/(α-Fe(Al) layer). The oxidation at 1000 ℃ changed the coating to ((α-Al₂O₃, Fe₂O₃)-mixed layer))/(AlFe₃ layer)/(α-Fe(Al) layer).