Thanks to polyol process, undoped and Co-doped ZnO nanoparticles (NPs) were successfully synthesized. Zn(NO3)2.6H2O was used as a precursor and Co(NO3)2.6H2O was the dopant. The morphology, structure, crystalline size and optical properties of the undoped and Co doped ZnO were measured in detail. The incorporation of the Co2+ in ZnO matrix was confirmed by DRX, ICP and EDX. Optical characterization of the sample revealed that the band gap energy (Eg) decreased from 3.36 to 3.25 eV with an increase in Co2+ doping concentration. The effect of Co doping on the photocatalytic activity of ZnO NPs was studied. The results showed that photocatalytic activity of ZnO nanoparticles was enhanced remarkably by Co doping. In comparison with pure ZnO, Co-doped ZnO NPs displayed a strengthened photocatalytic performance under mild UV-light irradiation for the methylene blue (MB) photodegradation.