The key factor to improve the electrochemical performance of Li-O2 batteries is to find effective cathode catalysts to promote the oxygen reduction and oxygen evolution reactions. Herein, we report the synthesis of an effective cathode catalyst of ruthenium nanocrystals supported on carbon black substrate by a surfactant assisting method. The as-prepared ruthenium nanocrystals exhibited an excellent catalytic activity as cathodes in Li-O2 batteries with a high reversible capacity of about 9,800 mAh g(-1), a low charge-discharge over-potential (about 0.37 V), and an outstanding cycle performance up to 150 cycles (with a curtaining capacity of 1,000 mAh g(-1)). The electrochemical testing shows that ruthenium nanocrystals can significantly reduce the charge potential comparing to carbon black catalysts, which demonstrated that ruthenium based nanomaterials could be effective cathode catalysts for high performance lithium- O2 batteries.