Effects of the ratio of ionomer to catalyst on the performance of DMFC were studied. Pt-Ru and Pt supported on carbon black were used as catalysts, and Nafion(DuPont) as ionomer and electrolyte. With increasing ratio of ionomer, electrochemically active surface area was changed, methanol crossover was reduced, and catalyst-ionomer agglomerate size was also changed. These methanol crossover and size effect played an important role in enhancing performance of DMFC.