The reaction mechanisms of dirhodium-catalyzed [3 + 2] and [3 + 3] cycloaddition between enol diazoacetate and isoquinolinium methylide have been studied in detail using density functional theory and a solution-phase translational entropy model. The reaction starts with the formation of a metallic carbene intermediate first, from which two competing reaction channels of [3 + 2] and [3 + 3] cycloaddition take place. For CAT1-catalyzed reactions, the calculated activation free energy barriers for [3 + 3] and [3 + 2] cycloaddition reactions are 14.3 and 16.0 kcal mol^–1, respectively, which is in good agreement with the ratio of products. Both the steric and electronic effects have been considered for CAT2- and CAT3-catalyzed reactions, with which the ratio of products has also been rationalized.