Magnesium oxide (MgO) nanosheets and hydrogen peroxide (H2O2) are respectively employed as a photocatalyst and an oxidant to enhance the photocatalytic efficiency for photo-degradation of methylene blue (MB). During the photocatalytic process, highly-oxidizing magnesium dioxide (MgO2) is generated by reacting with H2O2 on the edge of MgO nanosheets, which is verified by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and high-resolution transmission electron microscopy (HR-TEM). The synergistic catalysis of H2O2, MgO2 (highly-oxidizing) and MgO (photocatalysis) has significantly improved the photocatalytic efficiency. The photocatalytic efficiency of MgO nanosheets with H2O2 under visible-light irradiation reaches 98.1%, which is 3.2 times greater than that without H2O2 under visible light (30.5%). Moreover, the photocatalytic efficiency is comparable with that of traditional photocatalysts, such as titanium dioxide (TiO2), graphitic carbon nitride (g-C3N4), etc. This study indicates that the synergistic effect of the homologous oxide catalyst (MgO) effectively improves photo-degradation efficiency via in-situ generating a highly-oxidizing metal peroxide (MgO2) during the photocatalytic process. (C) 2019 Elsevier Inc. All rights reserved.