How to enhance the reaction efficiency using greener methods is an important topic. In this work, the phase behavior of the reaction system of metathesis of ethyl oleate with ethene in compressed CO2 was studied at 308.15 and 323.15 K using the Peng-Robinson equation of state. The effect of the phase behavior on the reaction rate and equilibrium conversion was studied. It was demonstrated that addition Of CO2 in the reaction system could increase the reaction rate and equilibrium conversion considerably at suitable conditions where the solubility of the reactant in the vapor phase was low, while the solubility of the products was very high. However, at the condition where the solubility of the reactant and products were all high, the reaction rate was much slower. The mechanism for this interesting phenomenon was discussed in detail.