Friction stir processing (FSP) was applied to modify the surface of a hypereutectic alloy Al-30wt.%Si, the influence of FSP passes on the size and morphology of Si particles, the mechanical properties of the materials in the stir zone was investigated. In the nugget by FSP, the silicon particles were intensely refined to several micrometers and spheroidized, and the refinement and spheroidization degree increased with FSP passes. The tensile behavior of the alloy by FSP has typically plastic feature. With FSP passes increasing, the tensile strength and the elongation increased gradually, and the improvement ratio of the elongation is more obvious. The improvement is attributed to the spheroidization and refinement of silicon particles by FSP.