We prepared dispersions of composite particles of thiacarbocyanine dye and colloidal silver or gold and examined an effect of the composite on the third-order nonlinear susceptibility (chi((3))) measured by using degenerate four-wave mixing method. The chi((3)) of the dispersion was enhanced about 3.5 times the sum of those of dye solution and colloidal metal dispersion. A new absorption band (lambda=600 nm) which was bathochromically shifted from monomer band (lambda=551 nm) was formed and this band was related to the enhancement of chi((3)).