In this study, pure cadmium oxide (CdO) nanoparticles as well as a mixed gadolinium oxide (Gd2O3)/CdO nanocomposites with mixing contents (0% ≤ Gd2O3 content ≤ 15%) were synthesized via facile precipitation technique followed by calcined treatment. X-ray diffraction analysis illustrated that all samples have high cubic phase purity and well crystalline nature. The as-synthesized CdO nanoparticles exhibited only the expected Bragg peaks for a single-phase cubic crystal structure of pure CdO. The Gd2O3/CdO mixed oxide nanocomposites gave rise to a Gd2O3 cubic phase as well the cubic phase of CdO. Further support for the formation of highly pure CdO and Gd2O3 phases was obtained via infrared (IR) spectroscopy. The morphologies of pure CdO and mixed Gd2O3/CdO nanoparticles were probed by scanning electron microscope and transmission electron microscope, which demonstrated that the collected samples consist of individual semi-spherical shaped entities with different particle sizes (23-31 nm). The optical band gap (Eg) of the developed samples was computed based on Tauc equation where it shows a decrease from 3.41 eV to 2.75 eV upon increasing the Gd2O3 content. 3D fluorescence analysis showed that, the quenching in the emission peak intensity with increasing Gd2O3 content is due to the high separation efficiency of photogenerated electron-hole pairs. Moreover, the Gd2O3(15%)/CdO nanocomposite shows superior photodegradation efficiency (89.1%) of methylene blue (MB) compared to 44.4% when using pure CdO. At pH 11.5, >3 times enhancement in degradation rate (0.044 min-1) is obtained compared to natural pH 9.6. Reusability study shows the stability of the Gd2O3/CdO photocatalyst in MB degradation for four cycles. Trapping experiment of holes and electrons revealed extensive contribution of holes rather than electrons in breeding of active oxidizing species.