A new sulfonated poly(aromatic imide-co-aliphatic imide) (SPI) for the use as a polymer electrolyte membrane was successfully synthesized from 4,4'-diaminodiphenylmethane (DDM), 4,4'-diaminodiphenylmethane-2,2'-disulfonic acid disodium salt (S-DDM), hexamethylenediamine, and 3,3',4,4'-benzophenonetetracarboxylic dianhydride in a one-step reaction. S-DDM was prepared by the direct sulfonation of DDM monomer. The degree of sulfonation of the sulfonated copolyimide was varied by using various molar concentrations of S-DDM. The sulfonated copolyimide with the highest degree of sulfonation possessed the proton conductivity of 0.0032 S.cm -1 in a wet state. The highest methanol permeability of the sulfonated copolyimide was 2.75 × 10 -8 cm 2 .s -1 which is ~425 times lower than that of the Nafion 117. The highest membrane selectivity belonged to the membrane with 34% degree of sulfonation with the value of 1.65 × 10 6 s.S.cm -3 which is three orders of magnitude higher than the commercial Nafion 117.