A multiscale model of lithium air batteries considering cathode pore size distribution is proposed, where the morphology of the discharge product, Li2O2, is assumed to be thin films covering the surface of the pores. In the model, active surface area degrades during discharge because of three reasons. First, the effective radius of pores decreases due to Li2O2 coverage. Secondly, small pores may be fully choked. Thirdly, thick Li2O2 film may block the electron tunneling process, rendering the surface inactive. Simulation results reveal that the end of discharge in cells made of Super P and Ketjen Black carbons is caused by unavailable surface area near the air inlet, rather than the full choking of pores. Larger discharge capacity is found in the Ketjen Black cell because its high specific surface area leads to slower Li2O2 thickness growth rate. We compare this tunneling-limited model with a linear resistance model where the Li2O2 thin film resistance is assumed to be proportional to its thickness. Different shapes of discharge curves have been discovered: the former has a long discharge plateau followed by a sudden. drop of cell voltage, while the latter shows a gradual decrease of cell voltage. These results are discussed in relation to the experimental knowledge. (C) 2014 The Electrochemical Society. All rights reserved.