Durability of perfluorosulfonic acid (PFSA) membrane is an important factor to improve the lifetime of PEFC. In this study the bond dissociation energy of PFSA molecule was investigated by quantum chemistry method to provide basic information for designing more durable PFSA membrane. Nafion® was used as a PFSA molecule. All calculations were performed by DMol3. Chemical bond strengths of PFSA molecule were analyzed exhaustively. The vulnerable bond was identified by comparing the results. The same calculations were performed for ionized PFSA molecule to find out the influence of the ionization state on the bond strength. Moreover, the possibility for improving durability of PFSA membrane was investigated by substituting H atoms or CH3 groups for a part of F atoms.