Rechargeable lithium-oxygen (Li-O2) batteries have received much attention because of their ultra-high theoretical energy density, about ten-times higher than that of current lithium-ion batteries.[1-4] However, the development of these batteries is still at its initial stage, owing to various technical challenges such as high overpotentials, low round-trip efficiency, poor rate capability, and low cycle life. Among various issues affecting Li-O2 battery performance, the low performance of the oxygen cathode has been identified as the dominating factor.[5, 6] The development of efficient cathode with nano-architectures, large accessible surface area, and bifunctional catalytic activity is a great challenge for high-performance lithium-air batteries.[7] In the present study, binder-free high surface area porous carbon nanofibers modified with cobalt nanoparticles (Co-PCNF) are presented as efficient cathodes for Li-O2 batteries. The macroporous inter-connected structure of carbon nanofibers with effective dispersion of cobalt nanoparticles serve as an efficient cathode with high electrocatalytic activity. Co-PCNF demonstrated much better performance as cathodes in Li-O2 batteries, with high discharge capacity, rechargeability, and rate capability, compared to non-cobalt-carbon cathodes. Li-air cells with Co-PCNF as cathode exhibit a high initial discharge capacity of 8800 mAhg-1 at the current density of 100 mA g-1, and can be recharged for more than 50 cycles with limited discharge capacity of 500 mAh g-1. Moreover, the synergistic role of N-and F- doped carbon defects and cobalt nanoparticles leads to faster electronic transfer and stabilize carbon surface, enhancing the reversibility.