The g-C3N4–ZnO composite photocatalysts with various weight percents of ZnO were synthsized by a simple calcination process. The photocatalysts were characterized by powder X-ray diffraction (PXRD), scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HR-TEM), UV-vis diffuse reflection spectroscopy (UV-vis), X-ray photoelectron spectroscopy (XPS) and thermogravimetric analysis (TGA). The PXRD and HR-TEM results show that the composite materials consist of hexagonal wurzite phase ZnO and g-C3N4. The solid-state UV-vis diffuse reflection spectra show that the absorption edge of the composite materials shifts toward the lower energy region and to longer wavelengths in comparison with pure ZnO and g-C3N4. Remarkably, the photocatalytic activity of g-C3N4–ZnO composites has been demonstrated, via photodegradation of Methyl Orange (MO) and p-nitrophenol experiments. The photocatalytic activity of g-C3N4–ZnO for photodegradation of Methyl Orange and p-nitrophenol under visible light irradiation was increased by over 3 and 6 times, respectively, to be much higher than that of single-phase g-C3N4, clearly demonstrating a synergistic effect between ZnO and g-C3N4. The concentrations of Zn2+ in g-C3N4–ZnO system after a photocatalytic reaction at various reaction times were found to be much lower than those for a ZnO system under the same reaction conditions, indicating that the g-C3N4–ZnO composite possesses excellent long-term stability for a photocatalytic reaction in aqueous solutions. Furthermore, a synergistic photocatalysis mechanism between ZnO and g-C3N4 was proposed based on the photodegradation results. Such obviously improved performance of g-C3N4–ZnO can be ascribed mainly to the enhancement of electron–hole separations at the interface of ZnO and g-C3N4.