This work reveals the grain refinement process of low-stacking fault energy Cu-4wt.%Ti alloy during surface mechanical attrition treatment (SMAT). Without phase transformations, the grain refinement process in Cu-4wt.%Ti alloy with a low stacking fault energy involves formation of planar dislocation arrays and twins in the small strain and low strain rate deformed region adjacent to the coarse grain matrix, twin-twin intersections leading to grain subdivision. The formation of lamellae, polygonal grains, and rotation recrystallization were induced by the large strain and high strain rate deformation near the treated surface. We also observed one distinct layer at the treated surface with the thickness about 15 µm, which is filled with equiaxed nanograins. The hardness of the treated surface was increased by 40% and attributed to the grain refinement according to the grain boundary strengthening mechanism.