Metal doped CdS nanoparticles were synthesized by a simple chemical precipitation route with different metals. The obtained nanoparticles were characterized by XRD and UV-vis reflectance spectroscopy. The results indicated that metal-doped CdS catalysts were successfully obtained with cubic structure and 4.0 - 4.5 nm crystallite size. The band gap energies of metal-doped CdS catalysts were estimated using UV-visible reflectance spectra to be about the range of 2.25 - 2.55 eV. Methylene blue was degraded by using metal doped CdS nanoparticles under a 400 W medium-pressure mercury lamp of visible light irradiation (λ > 420 nm). Higher degradation efficiency was achieved by adding metals to the photocatalyst compared with the single CdS catalyst. In this case, the degradation efficiency of Co-CdS catalyst after 4h irradiation time was about 87%.