Researchers normally consider MgO as a promising high-surface-area heterogeneous catalyst support, additive, and promoter for many kinds of chemical reactions due to its certain properties, including stoichiometry and composition, cation valence, redox properties, acid-base character, and crystal and electronic structure. The presence of MgO as a support catalyst also modifies the electronic state of the overall catalytic performance by electron transfer between the native catalyst and MgO as support. The influence is clarified by alteration of acid-base properties of the catalyst-supported MgO. Meanwhile, the method, chemical composition, and condition in the preparation of MgO are the important factors affecting its surface and catalytic properties. Therefore, MgO with a high surface area and nanocrystalline structure has encouraging applications for some reactions, including as dry reforming, dehydrohalogenation, oxidative dehydrogenation of butane, nonoxidative dehydrogenation of ethylbenzene, decomposition of CCl 4 , oxidative coupling of methane, hydrodesulfurization reactions, methane dimerization, and water gas shift reaction oxidation of CO by O 2 and ISO-propane conversion. This review highlights the synthesis and the role of MgO as a support catalyst and focuses on the design of better selective, energy-saving, durable, intrinsically clean, and safer catalytic processes.