In this work, a set of cylindrical rhodium coated micro channels with diameter range of 0.2–0.8 mm and a length of 1 cm is considered to simulate the behavior of the autothermal reforming reactions of methane to synthesis gas. A one-dimensional heterogeneous model is used to describe the flow field. Gas phase reactions are modeled by GRI 3.0 mechanism, and a 38-step detailed surface mechanism over a rhodium catalyst is considered for catalytic reactions. The effect of feed moisture content on product selectivity and yield, minimum required preheat temperature, length of the reforming and oxidation zones, and reactor thermal efficiency is studied at two different operating pressures of 1 and 20 atm, methane to oxygen feed ratios of 1.2–2.5, and pore diameters of 0.2–0.8 mm. Moreover, the simulation results are compared with the experimental data of Michael et al. [13].