In this paper, ordered mesoporous carbon (OMC)/graphitic carbon nitride (g-C3N4) composites were synthesized by a simple low-temperature thermal method. The structural properties of the OMC/g-C3N4 composites were characterized by XRD, N-2 adsorption desorption isotherms, and UV-vis diffuse reflectance spectroscopy. It was found that the addition of OMC slightly affected the graphitic stacking structures of g-C3N4, but strongly enlarged the surface area and total pore volume of OMC/g-C3N4 composites. The photodegradation of rhodamine B (RhB) and 2,4-dichlorophenol (2,4-DCP) under visible light irradiation was tested. The photocatalytic efficiencies of the 0.04-0MC/g-C3N4 composite over RhB and 2,4-DCP were about 2.41 times and 3.68 times as high as those of g-C3N4, respectively. It is deduced that the enhanced photocatalytic activity should be attributed to the larger specific surface area and favorable optical property.