Friction stir processing (FSP) is applied to modify the surface microstructure of cast AZ91 magnesium alloy. The electrochemical and hydrogen evolution measurements reveal that the corrosion rate of processed alloy in 3.5 wt % NaCl solution is significantly decreased. This is mainly attributed to the alteration of corrosion process induced by modification on the morphology and distribution of beta-Mg17Al12 phase via FSP. It is originally reported that the formation of a compact and continuous beta phase layer on the FSPed surface owing to the segregation of fine beta phase effectively enhances the stability and passivity of corrosion product film.