The effects of moisture contamination in the Li-O2 battery system were investigated by comparing the electrochemical performance and post-mortem analysis of batteries prepared under different atmospheres: sealed container in an ambient atmosphere vs. sealed container in a dry-room or in a glove-box. The performance of the cells strongly depended on the atmosphere; furthermore it was found that the performance degradation of the Li metal anode comes from moisture contamination from the feed lines. In an ambient atmosphere, the cells showed higher 1st discharge capacity, higher impedance and significant increase of the cell weight owing to contamination of the oxygen by moisture. Post-mortem analysis revealed that the deterioration of lithium metal anode leads to the cell failure mechanism, and this comes from the moisture contamination. It was found that the performance of Li-O2 batteries is very sensitive to even traces of moisture contamination and every single part of the cell design including the choice of fitting parts and water permeability of the fitting material should be verified in order to obtain credible and reproducible results. This finding supports the idea that protection of the lithium metal electrode is indispensable to realize the practical application of Li-O2 or Li-air batteries.