Formation of Ag-rich and Cu-rich Ag-Cu alloy nanoclusters in silica by Ag/Cu ion sequential implantation has been studied. The formation of alloy nanocluster has been evidenced by optical absorption spectra and transmission electron microscopy (TEM). Fast nonlinear refractive index and nonlinear absorption coefficient were measured at 790 nm of wavelength for Ag-Cu alloy nanocluster composites by the Z-scan technique. With different Ag/Cu ratio, linear absorption and nonlinear absorption are different in near infrared (NIR) region. It is suggested that by changing the ingredient percentage of metals in alloy, different optical nonlinearities could be selectively obtained.