Abstract: The coupled effect of photocatalytic activity of CdS/TiO₂ composite semiconductor was studied.CdS/TiO₂ nano-powders with different ratios of CdS∶TiO₂ from 0.005 to 2.0 were synthesized by the hydrothermal method at 200 ℃ for 6 h using titania, cadmium chloride and sodium sulfide as reactants. The CdS/TiO₂ powders were characterized by XRD, SEM, EDS, IR and the diameter of the particles was evaluated by Scherrer equation. XRD patterns showed that the crystallinestate of CdS/TiO₂ samples was a mixture of anatase, zinc-blend and wurtzite. SEM micrographs showed that the diameter of CdS particles was 20—50 nm and the diameter of TiO₂ particles was 50—100 nm. The photocatalytic activities of CdS/TiO2 nanopowders were measured by photo-degradation of rhodamine B. When the mole ratio of CdS∶TiO₂ was 1.0, the coupled effect of photocatalytic activities appeared.When the mole ratio of CdS∶TiO2 was 1.5, the coupled effect became the strongest, and the degradation rate of rhodamine B(4 mg•L-1) irradiated by ultraviolet ray for 30 minutes was 74.3%.