The nonlinear decoupling approximation is used to calculate numerically the effective linear dielectric function and third order nonlinear effect of metal particle composite media as function of random volume fraction,especially the high volume fraction,which show the nonlinear optimum volume ratio. The optical nonlinearity of composite media as functions of the frequency of the incident light under different volume fraction have been studied. It is found that near the surface plasmon resonance,optical nonlinearity increases when volume fraction inereases to certain value and then decrease due to the influence of the particle-particle interaction to the local field.