As one of the most promising energy-storage devices, supercapacitors have drawn great research interests because of their outstanding electrochemical behavior. Carbon paper holds great prospect as an electrode material for supercapacitors since it is inexpensive and highly conductive. In this paper, the modified Hummer's method is employed as an effective strategy to produce activated carbon paper with rougher surfaces and increased specific surface areas. It is further observed that hydrophilic groups such as hydroxyl (-OH), carbonyl (> C=O), and carboxyl (-COOH) have been introduced into the surface of carbon paper over treatment and they serve as strong polar sites to absorb water molecules, leading to improved hydrophilicity. Thus, the as-treated carbon papers with both increased specific surface areas and improved surface hydrophilicity exhibited excellent capacitive characteristics. Moreover, the charge/discharge test over 2000 cycles shows their excellent long-term capacitive retention.