We investigated the decarboxylation of 2-phenylcyclopent-3-ene-1,l-dicarboxylic acid (1) and its derivatives from the ab initio calculations. The reaction pathway starting from the R-configuration of 1 has been researched, and the transition states and endiol intermediates are computationally identified based on the imaginary frequency and its normal mode. The six membered ring transition states show the concerted mechanism for the C-C bond breaking and O-H bond formation. The intermediates are converted into the (R,R)- or (S,R)-2-phenylcyclopent-3-ene carboxylic acid. It has been found that the energy difference between two reactants (R1 and R2) is small (similar to1.1 kcal/mol) and is virtually the same as that between two transition states (TS1 and TS2), while the energy difference between the (R,R) and (S,R) isomers of the product is much larger (similar to7.39 kcal/mol). Thus, the stereochemistry of the product can be determined from the relative stability of the products as well as that of the transition states. The trans isomer is found to be favored due to its less steric hindrance than the cis isomer, which is consistent with previous experimental observation. However, the intramolecular interactions such as hydrogen bonding and pi-hydrogen interaction also play a role to a certain extent.