The electrochemical catalytic durability can be enhanced by the post treatment of diazonium reaction. The diazonium salts which have fluorine, acid, and nitrile functional groups were grafted on the surface of Pt based catalyst with 1–3 wt%. Pt/GKB6-Dia4F, which was a catalyst with fluorine functional group, showed the excellent properties in durability as well as in mass activity. After the durability test, electrochemical surface area (ECSA) and mass activity of the Pt/GKB6-Dia4F were maintained by 81.9% and 75.3%, respectively. These were higher than those of pristine catalyst, which were 60.8% and 58.9%, respectively. The causes of these enhancements were investigated in the aspect of electrostructure of platinum or wettability of the catalyst. In XPS, the valid Pt4f peak shifts of the treated-catalysts were not detected, but the indisputable changes of the surface properties via diazonium reaction were detected. This indicates that the main factor affecting the catalytic durability is the wettability of the catalyst. Therefore, we conclude that diazonium reaction is an effective method to change the surface property of the catalyst, which can enhance the catalytic durability.