In this paper, we employ the sum-over-states formula to calculate the dynamic third-order nonlinear optical polarizabilities of three optical processes, third-harmonic generation, electric-field-induced second-harmonic generation, and degenerate four-wave mixing, at the ground state of aniline oligomers. The ground state configurations of these compounds are obtained by the geometrical optimizations based on the B3LYP/6-31G** level. The transition moment elements and the transition energies are obtained by using the time-dependent density functional theory at the B3LYP/6-31G* level. The calculated results show that the third-order polarizitbility at the ground singlet state is increasing with the increase of repeat units number N of aniline oligomers and it approaches saturation at chain length around N = 7-8. (c) 2006 Elsevier Ltd. All rights reserved.