High silicon Al-Si alloy powders having nanocrystalline structures have been produced by mechanical alloying process. Microstructures in mechanically alloyed Al-Si powders were investigated by scanning electron microscopy and transmission electron microscopy. X-ray diffraction analyses were also carried out to characterize lattice constant, crystallite size and misfit strain. Effective milling time for the formation of nanocrystalline microstructure was thought to be approximately 12 hours, and the sizes of Al and Si crystallites in mechanically alloyed powders after longer than 12 hours of milling were reduced to about 30nm and 70nm respectively, in Al-70 mass% Si alloy system. The misfit strains increased with milling time up to 240 hours, and saturated to 5.73×10-3 and 4.39×10-3 for Al and Si crystallites, respectively.