Nanostructured composite fibers consisting of carbon coated Mn3O4 nanoparticles (Mn3O4@C) were prepared from thermal decomposition of manganese alginate fibers produced by wet-spinning technique, and investigated with SEM, TEM, XRD, nitrogen adsorption-desorption isotherms, and electrochemical tests toward energy storage. It is found that the as-obtained Mn304@C fibers consist of plenty of nano-sized Mn3O4 crystals with even diameter of 10-15 nm and carbon coating layer with a thickness of 1-2 nm. The composite fibers exhibit also a porous structure consisting of both micropores and mesopores. The electrochemical performances of Mn3O4@C fibers were examined by cyclic voltammetry and galvanostatic charge-discharge techniques. The results indicate that Mn3O4@C fibers possess a higher specific capacitance and superior rate capability when used as electrode materials for supercapacitor compared with commercial Mn3O@4. The improved performances of Mn3O4C fibers can be attributed to the nano-dimension of Mn3O4 particles, the thin carbon coating layer and the nanopores existing among Mn304@C nanoparticles.