Polyurethane open cell (PUOC) composites containing SiO2 and Al2O3 nanoparticles (NPs) were prepared. Scanning electron microscopy and Z-scan methods were used for observing porosity and detecting third-order nonlinear optical properties of related samples. Adding NPs into polymer matrix decreased the cell size and subsequently increased the porosity of samples. The nonlinear effects of samples were increased by adding 1 wt% of NPs into polymer in comparison with blanks. However, those features were decreased again through higher loading (up to 2.0 wt%) of NPs. The nonlinear refractive indices and nonlinear absorption coefficients of the synthesized samples were obtained in the order of 10−8 (cm2/W) with negative sign and 10−5 (cm/W), respectively. All the results suggest that the nonlinear coefficients of the synthesized samples can be controlled by NP contents in PUOC.