A base-catalyzed hydrolysis reaction of thiolester has been studied in both gas and solution phases using two ab initio quantum mechanics calculations such as Gaussian09 and CPMD. The free-energy surface along the reaction path is also constructed using a configuration sampling technique, namely, the metadynamics method. While there are two different reaction paths obtained for the potential profile of the base-catalyzed hydrolysis reaction for thiolester in the gas phase, a triple-well reaction path is computed for the reaction in the solution phase by two quantum mechanics calculations. Unlike the S(N)2 mechanism (a concerted mechanism) found for the gas-phase reaction, a nucleophilic attack from the hydroxide ion on the carbonyl carbon to yield a tetrahedral intermediate (a stepwise mechanism) is observed for the solution-phase reaction. Moreover, the energy profiles computed by these two theoretical calculations are found to be very comparable with those determined experimentally.