In this study, high silicon ductile iron alloyed with molybdenum was aluminized by hotdipping into a pure aluminum bath at 700℃. The effect of silicon and Mo-rich carbides in the steel substrate on the growth behavior of aluminide coating was studied. The results showed that the aluminide coating consisted of three layers, Al outer layer, Fe-Al intermetallic layer and Si pile-up layer, from the coating surface to the substrate. The formation of Si pile-up layers after hot-dipping and the existences of Mo-rich carbides and laminated pearlite in the substrate have the same effect of reducing the growth of Fe-Al intermetallic layer. As the hot-dipping time increased, the Si pile-up layers and Mo-rich carbides decomposed into Si particles and Mo carbides and then flowed out from the steel substrate. This resulted in the Al outer layers possessed the larger thickness.