Porosity effect of carbon materials on the electrochemical performances as cathode materials in non-aqueous Li-oxygen cells has been investigated. A carbon aerogel material is chosen to represent mesoporosity and a couple of activated carbon materials are selected to represent microporosity. Carbon aerogel shows at least ten times higher discharge capacity (4155 mAh g−1) than activated carbons. It is found that as much as 61% of pore volume of carbon aerogel is filled with discharge products (Li2O2) while less than 18% of pore volume is occupied with Li2O2 at the end of discharge in activated carbons, indicating that mesopore is more effective than micropore. In another experiments, the electrolyte impregnated electrode shows a capacitor-like discharge behavior, meaning it is necessary to have some open pore for the oxygen supply even though they are micropores.