Two kinds of ABC triblock copolymer, i.e. polystyrene-b-poly(hydroxyethyl acrylate)-b-poly(styrene sulfonic acid), (PS-b-PHEA-b-PSSA) at 23:23:54 wt. ratio and polystyrene-b-poly(hydroxyethyl acrylate)-b-poly(sulfopropyl methacrylate), (PS-b-PHEA-b-PSPMA) at 17:17:66 wt. ratio were sequentially synthesized via atom transfer radical polymerization (ATRP) for polymer electrolyte membranes fuel cells (PEMFC). However, the synthesis of polystyrene-b-poly(hydroxyethyl acrylate)-b-poly(2-methyl-2-propene-1-sulfonic acid), (PS-b-PHEA-b-PMPSA) was not successful. The middle PHEA block was crosslinked by sulfosuccinic acid (SA) via the esterification reaction between -OH of PHEA and -COOH of SA, as revealed by FT-IR spectroscopy. As increasing amounts of SA, ion exchange capacity (IEC) continuously increased for both membranes but water uptake decreased, presumably because of the competitive effect between crosslinked structure and the increasing concentrations of sulfonic acid group. As a result, the proton conductivities of crosslinked triblock copolymer membranes increased up to 0.049 and 0.037 S/cm at room temperature, for PS-b-PHEA-b-PSPMA and PS-b-PHEA-b-PSSA, respectively, with increasing SA weight fraction up to 0.038, above which they monotonously decreased. It was also found that the IEC value, water uptake and proton conductivity of PS-b-PHEA-b-PSPMA membranes were always higher than those of PS-b-PHEA-b-PSSA membranes, mostly due to higher concentrations of sulfonic acid groups in the former.