Systematic experiments were carried out to study the effects of various operating conditions on the performances of a direct methanol fuel cell (DMFC) using Nafion 117 and its modified membranes. The cell performance was studied as a function of cell operating temperature, methanol concentration, methanol flow rate, oxygen flow rate, and methanol-to-oxygen stoichiometric ratio. The experimental results revealed that the most significant factor was the temperature, increasing the cell performance from 50 to 80 °C. We achieved the maximum power density (Pmax) of 86.4 mW cm−2 for a DMFC at 80 °C fed with 1 M methanol (flow rate of 2 ml min−1) and humidified oxygen (80 ml min−1). A methanol concentration of 1 M gave much better performance than using 3 M of methanol solution. The oxygen and methanol flow rates with the same stoichiometric ratio had a beneficial effect on cell performance up to certain values, beyond which further increase in flow rate had limited effect. The Voc using argon plasma-modified Nafion was higher than the pristine Nafion membrane for the cell operated on 3 M methanol solution, which was due to the lower methanol permeability of the Ar-modified Nafion.