We have investigated third-order nonlinear optical properties of charge-transfer (CT) complexes of C-60 with various aromatic amines with different ionization potentials. The third-order nonlinear optical susceptibility \chi((3))\ measured by degenerate four-wave mixing shows a resonant enhancement at the absorption band due to the CT transition. We have found that the second hyperpolarizability gamma of CT complexes increases with an increase of the oscillator strength, which depends on the ionization potential. The highest hyperpolarizability among CT complexes studied here is 5.2 x 10(-28) esu for C-60/aniline CT complex.