The third- order nonlinear optical properties of two kinds of fullerenes (molecules of C60 and C70), which are solved in toluene, were studied through a degenerate four wave mixing process respectively. The third-order hyperpolarizabilities γ1111 of C70 molecule and C60 molecule are measured to be 1.2×10-30 esu and 4.0×10-31 esu, corresponding to an estimated third-order optical susceptibility x1111(3) of 2.5×10-8 esu and 8.5×10-9 esu for solid-state materials, respectively. The origin of the large nonlinear optical response of fullerene family is explained successfully with the free electron model.