To improve the photodegradation of methyl blue (MB) by ZnO under visible light irradiation, the sol-gel method was used to synthesize ZnO photocatalysts co-doped with Mg and supported on activated carbon (AC) with various properties. The photocatalysts synthesized were characterized in terms of their structure and optical properties. X-ray diffraction and UV-vis absorption spectra measurements were carried out to examine the structural and optical properties of photocatalysts. Such photocatalytic behaviors of photocatalysts preceded the order: Mg:ZnO/AC > ZnO/AC > ZnO. This consequence was ascribed to the small crystallite size, large surface area, the narrow bandgap of Mg:ZnO/AC photocatalyst. Besides, first-order kinetics accompanied the degradation of MB on photocatalysis. The active species (OH)-O-center dot and O-center dot(2)- performed more key roles in composite photocatalytic degradation of MB.