The main objective of this study is to utilize the synthesized CeO2/ZrO2 nanocomposite (CZ-NPs) in different concentrations (0.25, 0.5, 0.75, 1, and 1.5%) for improving the mechanical characteristics, microstructural properties, thermal resistance, and gamma radiation shielding of hardened ordinary Portland cement (OPC) composites. CZ-NPs were fabricated via the precipitation method. Various analytical techniques in addition to X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), Brunauer-Emmett-Teller (BET), and Scanning electron microscopy (SEM) analysis were used to investigate the fabricated CZ-NPs and hardened composite pastes. The physico-mechanical features, thermal stability, and radiation resistivity of different OPC-CZ-NPs composite pastes were investigated. The results showed that the composite CZNP1 containing 0.25% CZ-NPs is the best choice for mechanical performance and has the highest gamma-ray shielding efficiency, as it can attenuate 50–90% respectively of gamma rays with energy 660Kev at distances 4 cm and 13 cm, respectively. Also, the composite accommodating of 0.5% CZ-NPs motivates the thermal resistance at 400 °C.