Rechargeable lithium-oxygen (Li-O2) batteries have appeal to enormous attention because they demonstrate higher energy density than the state-of-the-art Li-ion batteries. Whereas, their practical application is impeded by several challenging problems, such as the low energy round trip efficiencies and the insufficient cycle life, due to the cathode passivation caused by the accumulation of discharge products. Developing efficient catalyst for oxygen reduction and evolution reactions is effective to reduce the overpotentials in Li-O2 cells. In our work, we report a Co3O4 modified Ag/g-C3N4 nanocomposite as a bifunctional cathode catalyst for Li-O2 cells. The g-C3N4 substrate prevents the accumulation of Ag and Co3O4 nanoparticles and the presence of Ag NPs improves the surface area of g-C3N4 and electronic conductivity, significantly improving the oxygen reduction/evolution capabilities of Co3O4. Due to a synergetic effect, the Ag/g-C3N4/Co3O4 nanocomposite demonstrates a higher catalytic activity than each individual constituent of Co3O4 or Ag/g-C3N4 for the ORR/OER on as catalysts in Li-O2 cells. As a result, the Ag/g-C3N4/Co3O4 composite shows impressive electrochemical performance in a Li-O2 battery, including high discharge capacity, small gap between charge and discharge potential, and high cycling stability.