The wavelength dispersions of third-order nonlinear optical response for Cu nanoparticle materials have been experimentally evaluated from transient spectra measured with the pump-probe method. The evaluated dispersions were analyzed on hot electron contribution using the Maxwell-Garnett approximation with the Drude model for intraband transition and first principles calculation for interband transition. The wavelength dispersion didn't directly reflect the dispersion of a local electric field factor. The interband transition term in hot electron contribution strongly dominated the dispersion around the surface Plasmon resonance by Fermi smearing. Intrinsic interband contribution to the nonlinearity was suggested from the analysis. Particle-size and host-medium dependence of the nonlinearity were also simulated.