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The morphology and microstructure of hypereutectic Al-Si alloy powder were analysized as a function of particle size using scanning electron microscope, differential scanning calorimetry and X-ray diffraction. The cooling rate and undercooling of powder was calculated. The results show that the homogeneousity of distribution of Si phases decreases with the decrease of particle size. Size of primary Si phase decreases with the decrease of particle size and the morphology of primary Si phase becomes regular. The morphology of eutectic Si changes into a network structure. X-ray diffraction line becomes boarded and shifts with the decrease of particle size. The DSC curve features of powders with different sizes deviate at the same time. The calculated results indicate that cooling rate and undercooling increases with the decrease of particle size, which leads to the deviation of microstructure.