This paper describes characteristics of passive direct methanol fuel cell (DMFC), especially the effect of the thickness of electrolyte membrane. Cell performances were compared for membrane electrode assemblies (MEAs) made with Nafion 112, 115, and 117, whose typical thicknesses were 51 µm, 127 µm, and 183 µm, respectively. The MEA made with Nafion 112 showed performance inferior to the other MEAs with thicker membranes. It appears that the mass of methanol permeated to the cathode through Nafion 112 was large, resulting in the loss of a large quantity of fuel, because its thickness was the smallest among the membranes used in this experiment. The observation of fuel level in the reservoir during open circuit stand indicated that most of the fuel was lost by methanol crossover and evaporation due to the rise in cell temperature. Also, the relationship between the concentration of methanol and the electrode potential at open circuit was investigated with a pseudo dynamic hydrogen electrode calibrated against an Ag/AgCl reference electrode. We found that it is essential to overcome the loss of fuel by methanol crossover and evaporation, which decreases the performance of the anode significantly even when the cell is in the stand-by mode.