Abstract: By means of the Fourier expansion technique and the decoupling approximation, the frequency dependence of the third order optical nonlinear susceptibility χ_e of composites with fractal geometry was ca lculated. The results show that obvious enhancement peaks occur in the Re(χ_{e)-ω curves at particular frequencies. Compared with the composites in which the inclusions were arranged in a periodic square lattice, when the fractal clusters of the inclusions were formed, the enhancement peak occurred in the case of periodic lattice was weakened, and new enhancement peaks occurred. The results also show that the intensities of the enhancement peaks were sensitively dependent on the characteristic relaxation time τ.}

Key words: composites, third order optical nonlinearity, fractal s