The reaction rate of the dimethyl carbonate (DMC) synthesis directly from carbon dioxide and methanol at a supercritical homogeneous state with a tin liquid catalyst is studied. Since the experimental time course for the reaction scheme has been already reported, the rate for the reverse reaction is obtained by a semi-batch reactor in this study. Together with previously reported data, the rate equations for the forward and the reverse reactions are modeled to express the dehydration effect. With the rate equation proposed, the overall reaction rate is fairly predicted. The rate equation is also applied to a flow type reactor with a simplified model. The reaction simulation shows that 90% dehydration for the reaction mixture makes the DMC yields three times larger than that for no dehydration. In addition, the pressure effect on the yield is investigated. On the contrary to linear pressure dependence of the DMC yield in the semi-batch reactor, the amount of DMC product from the flow type reactor changes slightly with the pressure. It suggests that the reaction pressure for the flow type reactor can be less than that for semi-batch reactor, showing the possibility of the industrialization of the process.