The size sudden change and structure evolvement of the macro-grains of AI-Si alloys with Si contents increasing were investigated in this article. It has been found that the curve for macrograin sizes versus Si contents takes the form of peak-valley. The macro-grain size of commercial pure Al is the coarsest and it will decrease greatly with Si increasing and get a valley near 3%Si, then it increases gradually with Si increasing and reaches the peak value near the eutectic point, the macrograin sizes of eutectic Al-Si alloy are coarse almost as that of pure Al. The macro-grain size of hypereutectic Al-Si alloys again decreases markedly with Si increasing. It is also found that the phase structures of the macro-grains of Al-Si alloys evolve gradually with Si increasing. The process of evolvement is described as: single alpha -Al phase --> alpha -Al phase surrounded by eutectic structures single eutectic structure --> primary Si crystals orderly surrounded by secondary alpha-Al phase and eutectic structures.