The recently reported Rh(II)-catalyzed direct C-H bond activation and lactonization of 2-arylphenols uncovers an attractive strategy to prepare coumarin derivatives with high site-selectivity. Motivated by the mechanistic ambiguity (on the origin of the site-selectivity and the details for lactonization etc.), we conducted a detailed mechanistic study of the rhodium-catalyzed lactonization of 2-arylphenols with density functional theory (DFT) calculations. The results suggest that the reaction occurs via the coordination exchange, C-H bond activation, carboxylation, protonation and lactonization steps. The rate-determining step is the carboxylation step, in which CO2 favorably inserts into the Rh-C bond (instead of the more nucleophilic Rh-O bond). The protonation step after carboxylation is critical, which makes the subsequent CO2-assisted lactonization feasible. Interestingly, the corresponding pKa value of the base can reasonably predict the reaction energy barrier of the C-H bond activation step. The calculations will provide insights and suggestions for the development and advancement of the subsequent C-H bond activation carboxylation reaction.