Methane steam reforming has been carried out in a Pd-Ru membrane reactor at 500∼600℃. The membrane reactor consisted of a Pu-6%Ru tube of 100㎛ wall thickness and commercial catalysts packed outside of the membrane. The methane conversion was significantly enhanced in the membrane reactor in which reaction equilibrium was shifted by selective permeation of hydrogen through the membrane. The methane conversion at 500℃ was improved as high as 80% in the membrane reactor, while equilibrium conversion in a fixed-bed reactor was 57%. The effect of gas flow rate and temperature on the performance of the membrane reactor was investigated and the results were compared with the simulated result from the model. The model prediction is in good agreement with the experimental result. In order to apply the membrane in practice, however the thickness of the membrane has to be reduced. Therefore, the effect of membrane thickness on performance of the membrane reactor was estimated using the model.