The development of efficient cathodes is a great challenge inhibiting the advancement of lithium-oxygen (Li-O-2) batteries. In the present study, binder-free, high surface area hierarchically-porous carbon nanofibers decorated with cobalt nanoparticles (Co-PCNF) are investigated as cathodes for Li-O-2 batteries. We fabricate the nanofibers using a facile electrospinning technique followed by thermal treatment with in situ incorporation of cobalt nanoparticles. This method provides a free-standing, electron-conducting network with a hierarchical pore structure and effective dispersion of cobalt nanoparticles, which is directly used as a cathode in Li-O-2 cells without any binders. Li-O-2 cells with Co-PCNF as the cathode exhibit a high initial discharge capacity of 8800 mA h g(-1) at the current density of 100 mA g(-1), and can be recharged for more than 50 cycles with a limited discharge capacity of 500 mA h g(-1). In comparison, porous carbon nanofibers without cobalt provide a discharge capacity of 6670 mA h g(-1), and a cycle life of only 35 cycles. The post mortem analysis of discharged cathodes revealed Li2O2 as the major discharge product, and suggested a LiO2-mediated reaction mechanism responsible for the excellent performance of Co-PCNF.