A fiber-type Na-Mn-O cathode for use in a sodium ion battery was studied in terms of its electrode performance and structural characterization. For preparing the cathode, a Mn-O precursor layer was first electrodeposited onto carbon fibers, followed by a hydrothermal reaction with NaOH to form the Na-Mn-O active material. The cathode's properties were investigated based on electrochemical characterizations and its structure was analyzed based on a synchrotron X-ray diffraction analysis. The synthesized orthorhombic Na0.91MnO2 phase was found to irreversibly change to themonoclinic NaMnO2 phase along with unit cell deformation and volume change during the 1st cycle discharge, resulting in an initial discharge capacity of 205 mAh/g. This suggests that this fiber-type cathode possesses an effective current-collecting performance without extra conductive material and binder. In addition, side reactions caused by the additives can be avoided, thus contributing to a lower polarization during the charge/discharge cycling. It was also found that the capacity fade during cycling was attributed to a partial phase change from the NaMnO2 phase to the inactive Mn-O phases. (C) 2014 The Electrochemical Society. All rights reserved.