In fact, there is a lack of corresponding studies of the effect of CeO2 nanoparticles on different features of hardened cement pastes. Therefore, the main objective of this current study is to investigate the performance of low-cost CeO2 nanoparticles on the physico-mechanical, and thermal treatment properties of Ordinary Portland Cement (OPC). Cerium oxide nanoparticles (CNP) were prepared via the precipitation method followed by calcination at 500 °C for 2 h. The fabricated (CNP) has been studied by using various investigation tools such as XRD, FT-IR, TEM, SEM, DTA/TGA, and BET surface area. Various doses of CNP (0.0, 0.25, 0.5, 0.75, 1.0, and 1.5 wt%) were added to the OPC. The mechanical, physical and thermal stability of OPC–CNP hardened composites were investigated. The positive collaborative of fabricated CNP with an average crystallite size of 13 nm was confirmed by XRD and TGA/DTG methods, which indicated that the addition of 0.25% CeO2 nanoparticles (CNP) to OPC pastes enhanced the compressive strength at 90 days. The compressive strength of CNP4 and CNP5 (1.0 and 1.5% wt.%) showed the higher values at all thermally treatment temperature up to 1000 °C. All nano-CeO2 composite pastes exhibited the high photocatalytic activity (80.0–84.0%) for the degradation efficiency of methyl orange (M.O) dye under UV irradiation in λ = 300 min.