The effect of various nanoclay loading levels on the properties of unsaturated polyester/clay nanocomposites was investigated. Nanocomposites containing 1, 3, 5, and 7 wt% organically modified nanoclay were prepared via in situ polymerization. Success of the polymerization reaction was demonstrated by the FTIR spectra. The morphological structure of the samples was examined by XRD measurements. Moreover, TEM micrograph of one of the samples was also studied. Density and water absorption measurements revealed that the incorporation of the nanoclay to the polymeric matrix could reduce the volume shrinkage of UP resin during the curing process and could improve the resistance of the material against water diffusion. DSC analysis showed that the initiation temperature and the curing temperature range decreased after the addition of nanoclay. Improvement in the thermal and corrosion resistance of the nanocomposites compared to the pure UP polymer was shown by TGA and electrochemical resistance measurements, respectively, and the best properties were observed for the nanocomposite containing 5 wt% nanoclay. Finally, the effect of electron beam irradiation on the properties of the nanocomposite samples was investigated. Irradiation with doses up to 500 kGy leads to an increase in water absorption resistance, corrosion resistance, and density of the nanocomposite. However, higher examined dose of irradiation (1000 kGy) had a deteriorating effect on the nanocomposite properties.