A soluble poly(2-methoxy-5-decoxy)-p-phenylene vinylene (PMODOPV) was synthesized via dehydrochlorination reaction and characterized by UV-visible absorption spectrum. The results showed that PMODOPV had a broad optical absorption band in the range of 450---550 nm and the optical band gap was calculated to be around 2.12 eV. The third-order nonlinear optical susceptibilities (χ(3)) of PMODOPV films implanted by 30-keV N+ ions in the dose range of 3.8 * 10^(15)---9.6 * 10^(16) ions/cm2 were measured by degenerate four-wave mixing (DFWM) technique. The results demonstrated that the resonant and off-resonant χ(3) values were all enhanced significantly by means of ion implantation. The maximum resonant χ(3) value of 9.67 * 10^(-9) esu at 532 nm and off-resonant χ(3) value of 2.49 * 10^(-10) esu at 1.064 μm were obtained when the ion dose was 3.8 * 10^(16) ions/cm2.