The ultrafast third-order optical nonlinearity Of two C-60 derivatives was measured by using the optical Kerr effect (OKE) at 830 nm wavelength, with laser pulse duration of 115 fs. Tremendous difference in OKE responses was observed due to the different attached bond site. For the [5,6]azafulleriod, a supposition was suggested and the large OKE response of solution subtracting from the contribution of solvent was obtained and its second-order hyperpolarizability gamma was deduced as large as 2.9 x 10(-31) esu, which is three orders of magnitude larger than the value Of C-60. While, for the [6,6]azafullerene, no any enhancement is observed in comparing with that Of C-60. (C) 2004 Elsevier B.V. All rights reserved.