The direct synthesis of dimethyl carbonate (DMC) from methanol and carbon dioxide using potassium methoxide as a catalyst in the presence of CH3I was investigated. The results showed that CH3OK is an effective catalyst; the raised yield (16.2%) and high selectivity (100%) of DMC formation under mild conditions were achieved. The effect of the reaction conditions, such as reaction temperature, pressure of carbon dioxide, the molar ratio of potassium methoxide to methanol and the amount of methyl iodide, on the reaction is discussed. The roles of the potassium methoxide and methyl iodide were explored, that is, the former acts as a catalyst, and the latter does as a promoter to the formation of DMC from CH3OH and CO2. In addition, the reaction mechanism was proposed according to the results obtained in the catalytic reaction.