A transient model for passive direct methanol fuel cell (DMFC) is developed to investigate the effect of cell orientation and operating condition. The results show that the passive DMFC with vertical orientation has better performance than the horizontal one, except the case of high current density, because a large amount of water produced in cathode is hard to be removed in vertical orientation, which is easier for horizontal orientation due to gravity. The passive DMFC with horizontal orientation is sensitive to methanol crossover, and moderate current density or voltage is necessary to ensure high energy efficiency. The anode micro-porous layer (MPL) plays an important role in reducing the rate of methanol crossover by providing flow resistance. The MPL in cathode has a significant effect on water transport by enhancing the water back-flow from cathode to anode, which prevents water removal. Therefore, the anode MPL and cathode MPL have different effects on horizontal orientation and vertical orientation. Additionally, the size of fuel tank can improve the energy density by providing more fuel, and the effect on fuel efficiency and energy efficiency is a bit obvious in vertical orientation than horizontal orientation. Copyright (C) 2016, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.