Poly(2,5-didecyloxy)phenylene (PDDOP) films implanted by 30 keV nitrogen ions (N +) with the dose rang of 10 15–10 17 ions/cm 2 were characterized by FTIR–ATR spectra, UV–VIS spectra and X-ray photoelectron spectroscopy, which showed that the C–H bonds of the PDDOP films were largely broken and new bonds like C C, C N and C N were formed as the increasing ion fluence. The third-order nonlinear optical susceptibilities ( χ (3)) were measured by degenerate four-wave mixing (DFWM) technique at 532 nm. The results demonstrated that the ( χ (3)) value of PDDOP films was maximized to 4.19 × 10 −11 esu at an ion dose of 1.15 × 10 17 ions/cm 2, which was 4.4 times larger than that of the pristine film. The enhanced third-order nonlinear properties may attribute to the enlarged expansion coefficient and the enhanced absorption coefficient of the bombarded films at proper N + ions dose.