The nonlinear optical anisotropy of ply(p-phenylene benzo-bisthiazole) (PBT) Langmuir-Blodgett (LB) films has been investigated by optical third-harmonic generation measurement. Polymer backbones of PBT in LB films were aligned along the dipping direction. The χ(3)xxxx, χ(3)yyyyand χ(3)xxyyvalues were determined by rotational Maker fringe method to be 11.3 × 10−12esu., 1.5 × 10−12esu. and 0.9 × 10−12esu, respectively. These nonlinear optical anisotropies can be explained well by assuming a Gaussian orientational distribution of one-dimensional π-conjugated chains.