Using electrospun carbon nanofiber (ECNF) as substrate, the porous MnOX-encapsuled ECNF composites (A-ECNF/MnOx) were fabricated via in-situ deposition of MnO2 followed by heat-treatment. The MnOX loading amount was tuned by the deposition time in dilute KMnO4 solution. The structure and surface properties of ECNF/MnOx composites before and after annealing treatment were characterized by scanning electron microscopy, transmission electron microscopy, X-ray diffraction, X-ray photo spectroscopy and contact angle test. The unique open porous surface structure of the fibers with mixed-valence Mn element has been realized by annealing treatment. The electrochemical properties of the annealed A-ECNF/MnOx composites have been greatly improved, with a specific capacitance as high as 200 F/g. With A-ECNF/MnOx composites as cathode in capacitive deionization (CDI) cell, the desalination amount of 8.2 mg/g and the charge efficiency of 70% were achieved, much higher than those of pristine ECNF and ECNF/MnOx without annealing, thanks to the ultrathin MnOx load, good hydrophilicity and the open porous surface of the composites. The A-ECNF/MnOx composite is a promising electrode material candidate for CDI application and for various electrochemical devices. (C) 2016 The Electrochemical Society. All rights reserved.