Friction stir processing (FSP) was applied to a cast aluminium alloy, A356-T6, and a cast magnesium alloy, AZ91-T5, and fully reversed axial fatigue tests have been performed using FSPed specimens. It was indicated that FCP exerted different influence on fatigue behaviour depending on alloy system. In A356-T6, the fatigue strengths of the FSPed specimens were lower than those of the as-cast ones in the finite life region, but the fatigue limit was significantly increased by FSP. The enhanced crack initiation resistance due to the elimination of casting defects by FCP resulted in the improvement of fatigue limit of the FSPed specimens, while the matrix softening due to the dissolution of precipitates by the heat input during FSP caused faster crack growth rates in the FSPed specimen, leading to the inferior fatigue strength of the FSPed specimens to the as-cast one in the finite life region. In AZ91-T5, both the fatigue strengths in the finite life region and fatigue limit were improved by FSP, because the hardness was increased and both the crack initiation and crack growth resistances were enhanced.