This paper reports a first-principles molecular dynamics simulation study of the microscopic mechanism of the base-catalyzed dehydrochlorination of pentachloroethane. So far the nature of the mechanism of this reaction is not understood: the concerted and stepwise mechanisms are under debate. By combining ab initio molecular dynamics with the method of constraints, we determine the reaction mechanism and associated free energy profile. We find that the reaction barrier is in good agreement with experimental findings and reveal that the reaction proceeds via a concerted mechanism. Our simulations provide no evidence for the presence of carbanion intermediate indicative of the stepwise pathway. This microscopic understanding will provide new implications for understanding the reduction of polyhalogenated alkanes and rational design of effective materials to treat these contaminants.