Effect of Au nanoparticles size on the third-order nonlinearity of PVA films is investigated experimentally. Three samples of Au nanoparticles with different sizes were produced by laser ablation method. They were used to produce Au nanoparticles doped PVA thin films. Using Z-scan technique, the nonlinear refractive index and nonlinear absorption coefficient of films were measured under the irradiation of the second harmonic of a low power CW Nd:YAG laser. Results show that by decreasing the size of nanoparticles in the range of 9–20 nm, the nonlinear refractive index of Au doped PVA films decrease while their nonlinear absorption coefficient increases. Thermo-optical coefficient of samples was also calculated. Increasing the size of nanoparticles leads to increase their thermo-optical coefficient.