In this paper, the polarization of an oxygen electrode in rechargeable lithium oxygen batteries was investigated and found to be closely related to the fading of discharge capacity and ceasing of cycling. Electrochemical tests showed that with an alpha-MnO2 catalyzed air electrode, the cell could deliver a high initial discharge capacity of 2300 mAh g(-1) of carbon with rapid fading to 150 mAh g(-1) at the fourth cycle. When the cell was cycled at a restricted depth of discharge, the cycle life could be prolonged. Electrochemical impedance spectra responses and scanning electron microscopy observations indicated that the discharged product could block the air pathway, increase electrochemical resistance, and lose electrical contact. To improve the electrochemical performance of lithium oxygen batteries, the oxygen reduction catalyst and the structure of the O-2 electrode deserve further investigation.