Abstract: The effect of reaction conditions on the gross heat efficiency were studied on the basis of mass,and energy, balance as well as the kinetics of POX in the on-board hydrogen generation process, and the suitable reaction conditions were optimized when the whole system was considered as an autothermal process. In order to get η as high as possible, the mol ratios of O ₂/CH ₃OH ( f ) and H ₂O/CH ₃OH ( n ) should be adjusted carefully, and the suitable f and n were ca. 0.35 and ca. 0.8 respectively. The effect of reaction pressure on η was relatively trivial, which made it possible for POX to take place at a relatively high pressure to meet with PEMFC which usually works at 0.3 - 0.5 MPa. The heat exchange efficiency between the reformate and the cool reactants in the evaporator should be enhanced by increasing the heat exchange areas in order to obtain higher heat efficiency. Although fairly good heat efficiency could still be obtained when more methanol was consumed to meet with the increase of loading of PEMFC, more oxygen would have to be provided to the whole system which made the ’hot spot’ temperature even worse.