The REE (rare-earth-elements) cerium (Ce) is the most abundant earth-crust element and their oxides have great attention in the form of nanocrystalline nature with superior physical and chemical properties. Pure and Co (1%, 3% and 5%) doped CeO2 nanoparticles (NPs) synthesized by co-precipitation technique were characterized through X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), UV-visible spectroscopy. XRD shows face-centred-cubic (FCC) crystal symmetry with average crystallite size 6-12 nm. HRTEM exhibits almost identical cubical shaped particles with average size 4-10 nm. Tuned band-gap may be observed from UV-visible spectrum of CeO2-NPs upon Co (1%, 3% 5%) incorporation. Enhancement of the photocatalytic activity observed for Co-doped (1%, 3% 5%) to the degradation of methylene-blue (MB) dye under visible-light absorption. © 2020 Materials Research Society.