Flexible porous carbon nanofibers containing nickel nanoparticles were synthesized by direct carbonization of electrospun Ni-MOFs/polyacrylonitrile fibers. The as-synthesized composite nanofibers were employed as binder-free electrodes, and exhibit high specific capacitance (up 672 F g-1 at current density of 2 A g-1) and superior rate capability (57% capacitance retention from current density of 2-10 A g-1), which may be attributed to their binder-free nature, unique one-dimensional (1D) structure and highly dispersed electrochemically active nickel nanoparticles. Furthermore, a symmetric supercapacitor was assembled using the fiber electrodes in 6 M KOH, and the energy density of 17.8 Wh kg-1 was achieved in a potential window of 1.5 V. This self-standing fiber with abundant mesopores and macropores is expected to become a promising electrode material for high-performance supercapacitors.