We report carbon nanotube-encapsulated cobalt as an efficient oxygen reduction electrocatalyst (onset potential of 0.94 V and half-potential of 0.84 V). Calculation results firstly reveal that Co protected by graphitic-N-doped carbon holds more negative charge (-1.22 eV) and has an energy barrier (0.56 eV) lower than that of pyridinic-N-doped carbon (-1.11 eV; 0.78 eV), which is responsible for the ORR activity. The corresponding Zn-air batteries deliver an excellent gravimetric energy density of 837 W h kg-1.