Using the ab initio method, the geometrical structures of C-36 and the X (B,N)-doped isomers C34X2 have been optimized. On the basis of the optimized structures, then, the third-order nonlinear optical polarizabilities gamma in the different optical processes of the third-harmonic generation, electric-field induced second-harmonic generation and degenerate four-wave mixing, and two-photon absorption (TPA) cross sections delta are calculated by using TDB3LYP method coupled with the sum-over-states method. The calculated results show that the one-photon allowed excitation process dominate the two-photon excitation process for C-36-D-6h, whereas the two-photon allowed excitation process dominate the one-photon excitation process for C-36-D-2d and C34X2 (B,N). It is found that the largest resonant TPA peaks of dopant fullerenes have a blueshift and the TPA cross sections have an enhancement compared with those of the parent fullerenes of isomers C-36-D-6h and C-36-D-2d. (C) 2004 American Institute of Physics.