The third‐order optical nonlinear susceptibilities of single‐crystalline and spin‐coated amorphous 4‐butoxy‐carbonyl‐methyl‐urethane polydiacetylene (red form) thin films were measured by third‐harmonic generation. The third‐harmonic intensities were calculated as a function of incident angle by solving the propagation equation of the electromagnetic field for a two‐layer system with one nonlinear media. From the best fit of the calculated and experimental data, the ( &khgr;(3)) values of the spin‐coated amorphous film (&khgr;(3)p) were found to be (9.6±1.0)×10−12esu and (2.0±0.2)×10−11esu at 1.064 and 1.319 &mgr;m, respectively, while the &khgr;(3)values of the crystalline ( &khgr;(3)pc) were (4.9±0.5)×10−11esu and (1.0±0.07)×10−10esu at 1.064 and 1.319 &mgr;m, respectively. A factor of five increase in the &khgr;(3)of the crystalline film is attributed to the orientation effect of the polymer chains. The enhancement of the nonlinearity at 1.319 &mgr;m may be due to either three‐photon or two‐photon resonant effects.