Sulfonic acid groups were grafted onto the surface of carbon-nanotube supported platinum (Pt/CNT) catalysts to increase platinum utilization in polymer electrolyte fuel cells (PEFCs) by both thermal decomposition of ammonium sulfate and in situ radical polymerization of 4-styrenesulfonate. The resultant sulfonated Pt/CNT catalysts were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), Fourier transform infrared (FTIR) spectrometry, thermal gravimetric analysis (TGA) and electrochemical methods. The electrodes with the Pt/CNT catalysts sulfonated by the in situ radical polymerization of 4-styrenesulfonate exhibited better performance than did those with the unsulfonated counterparts, mainly because of the easier access with protons and well dispersed distribution of the sulfonated Pt/CNT catalysts, indicating that sulfonation is an efficient approach to improve performance and reduce cost for the Pt/CNT-based PEFCs. The electrodes with the Pt/CNT catalysts sulfonated by the thermal decomposition of ammonium sulfate, however, did not yield the expected performance as in the case of carbon black supported platinum (Pt/C) catalysts, probably due to the significant agglomeration of platinum particles on the CNT surface at high temperatures, indicating that the Pt/CNT catalysts are more sensitive to temperature than the Pt/C catalysts.