A two-dimensional, transient, and non-isothermal model was developed in this work to study the mass transfer properties of the Li-air battery. Special attentions have been paid to the cathode carbon electrode and the distributions of oxygen, lithium ion, lithium peroxide, and temperature in the carbon electrode have been calculated in the model. The effects of discharge current, electrode thickness, porosity distribution in the electrode, and cathode open ratio on the discharge capacity of the battery have been investigated. Modeling results showed that the discharge capacity of a Li-air battery was primarily determined by the oxygen supply. Most of the available pores deep in the electrode were not utilized because Li2O2 accumulated at the electrode/air interface and blocked the oxygen. It was also found that the utilization rate of the electrode was lower when the electrode was thick, the cathode open ratio was low, and the discharge current was high. A unique design of the carbon electrode with a non-uniform porosity distribution was proposed, and the discharge capacity of the battery was increased by more than 25% after implementing the new electrode. (C) 2012 The Electrochemical Society. [DOI: 10.1149/2.043210jes] All rights reserved.