This paper was presented to determine the methanol crossover and efficiency of a direct methanol fuel cell (DMFC) under various operating conditions such as cell temperature, methanol concentration, methanol flow rate. cathode flow rate, and cathode backpressure The methanol crossover measurements were performed by measuring crossover current density at an open circuit using humidified nitrogen instead of air at the cathode and applied voltage with a power supply The membrane elm trode assembly (MEA) with an active area of 5 cm(2) was composed of a Nafion 117 membrane, a Pt-Ru (4 mg/cm(2)) anode catalyst, and a Pt (4 mg/cm(2)) cathode catalyst. It was shown that methanol crossover increased by increasing cell temperature, methanol concentration, methanol flow rate, cathode flow rate and decreasing cathode backpressure Also, it was revealed that the efficiency of the DMFC was closely related with methanol crossover, and significantly improved as the cell temperature and cathode backpressure increased and methanol concentration decreased (C) 2010 Elsevier Ltd. All rights reserved