A novel sulfonated diamine monomer, 4, 4'-bis (4-amino-phenoxy) biphenyl-3,3-disulfonic acid (BAPBDS), was synthesized by direct sulfonation of the corresponding parent diamine. Sulfonated polyimides were prepared from 1,4,5,8-naphthalenetetracarboxylic dianhydride (NTDA), BAPBDS and 2,2'-bis (trifluoromethyl) benzidine (TFMBz). BAPBDS-based polyimides exhibited the best water durability among the sulfonated polyimide membranes investigated so far. This is attributed to the flexible structure and the high basicity of BAPBDS. Proton conductivity of the sulfonated polyimides investigated was similar to that of Nafion 117 at 100% relative humidity. With decreasing relative humidity, the proton conductivity decreased. The rate of decrease was in the order BAPFDS > ODADS > BAPBDS-based polyimides. NTDA-BAPBDS/TFMBz(4/1)copolyimide membrane displayed good water durability and reasonably high conductivity and showed potential for fuel cell applications.