Abstract: The nanocrystalline structure was present on the surface of Cu-4.5Ti alloys after the surface mechanical grinding treatment (SMGT). The effect of aging time on the microstructure and hardness of Cu-4.5Ti alloys was studied by the X-ray diffraction (XRD) analysis, transmission electron microscopy (TEM), and microhardness test. In the results, the Cu-4.5Ti alloys after SMGT show the plastic deformation, and the surface plastic deformation is the most obvious; the nanocrystalline structure is found in the alloy samples, and many mechanical twins are generated. After 8 h aging treatment, the denser twin structures are formed in the alloy samples, and more twins regions are present. The microhardness of the Cu-4.5Ti alloys after SMGT increases first and then decreases from outside to inside, and finally reaches a stable state. The alloy samples after 8 h aging treatment reach the peak hardness, which is HV 213 at the surface and HV 308 at about 50 μm off the surface.