Hydrogen production through autothermal reforming (ATR) of hydrocarbons, such as methanol, is an attractive option for mobile applications of hydrogen fuel cells. This paper presents the result of numerical simulation of methanol autothermal reforming in a microreactor over CuO/Al2O3 catalyst. The microreactor is a single channel of 700 µm width and 10 mm length and wall thickness of 150 µm. Effects of gas hourly space velocity (GHSV), steam to carbon ratio (S/C), and oxygen to carbon ratio (O2/C) at the reformer inlet is investigated. Results from this investigation show that GHSV of 10,000, S/C of 1.0 and O2/C of 0.12 will lead to higher hydrogen concentrations at the outlet while keep the CO mole fraction less than 2%.