By using the decoupling aproximation and a series expression of the effective dielectric function ε., the frequency-dependent cubic nonlinear susceptibility X., of metal-insulator composites with periodic structure was strdied. The results show that if the nonlinear metal phase is in the form of coating on the linear insulator core particles. the nonlinear frequency response o the composite has one sharp peak near the surface plasmon response frequcncy of the metal. and. an enhancement of x. of several orders of magnitude can be produced. The result s also show that the nonlinear response of the composite is sensitive to the relaxation time of the metal and the dielectric constant of the linear insulator.