Friction stir processing (FSP) was applied to modify the surface of a hypereutectic Al-30wt.%Si alloy, the influence of the parameters on the size and morphology of Si particles, the microhardness of the mateials in the stir zone was investigated. All samples were preheated to research the influence of the pure heat input on Si. With the preheating temperature increasing from 100°C to 400°C, the average diameter increases gradually, but the shape factor decreases, the microhardness of the materials decreases. With the rotation rate increasing from 375rpm to 1050rpm, both the average diameter and shape factor increase, but the microhardness decreases. With the FSP passes increasing from 2 to 6, the average diameter decreases, and both the shape factor and the microhardness increases. Linear regression lines in most cases are presented to reveal the quantitative relationship between the FSP parameters and the Si particles and the microhardness. The reasons of the refinement and the spheroidization of the Si particles are carefully discussed. The microhardness variation of the materials is attributed to the size change of the Si particles with the FSP parameters.