In this study, activated carbon (AC) as a carbon source was modified with different concentrations of cobalt chloride (CoCl2) to prepare a Co-AC composite, and it was used for the preparation of Co-AC/TiO2 composites with titanium oxysulfate (TOS) as the titanium precursor. The physicochemical properties of the prepared Co-AC/TiO2 composites were characterized by N2 adsorption at 77 K, X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy dispersive X-ray (EDX) analysis. The photocatalytic treatments of organic dyes were examined under an irradiation of visible light with different irradiation times. N2 adsorption data showed that the composites had decreased surface area compared with the pristine AC, which was 389 m2/g. From the XRD results, the Co-AC/TiO2 composites contained a mixturephase structuresof anatase and rutile, but a cobalt oxide phase was not detected in the XRD pattern. The EDX results of the Co-AC/TiO2 composites confirmed the presence of various elements, namely, C, O, Ti, and Co. Subsequently, the decomposition of methylene orange (MO, C14H14N3NaO3S) and rhodamine B (Rh.B, C28H31ClN2O3) in an aqueous solution, respectively, showed the combined effects of an adsorption effect by AC and the photo degradation effect by TiO2. Especially, the Co particles in the Co-AC/TiO2 composites could enhance the photo degradation behaviors of TiO2 under visible light.