Disclosed is a method of preparing a metal oxide-graphene nanocomposite, including preparing a nanocomposite material, forming graphene flakes by pretreating the nanocomposite material, and hydrothermally synthesizing the pretreated nanocomposite material. A method of manufacturing an electrode using the metal oxide-graphene nanocomposite is also provided. According to this invention, the metal oxide-graphene nanocomposite is synthesized from inexpensive graphite through one-step processing using only a surfactant, in place of conventional methods using oxidants, reductants and high-temperature heat, thereby lowering the number of processing steps and processing costs. Also, in the fabrication of the electrode, low electrical resistance characteristic of graphene is applied as it is, in place of the conventional use of active material, conductive material and binder, thereby exhibiting desired processing efficiency without the addition of the conductive material. Furthermore, highly pure graphene is prepared in a short time and various metal oxide active materials suitable for use in energy storage devices, for example, unary, binary, and multicomponent metal oxides, is formed through one-step processing, and necessary oxides having desired weight ratios {cobalt oxide (CoO), tricobalt tetraoxide (Co3O4), and cobalt hydroxide [Co(OH)2]} can be easily prepared, and thus very wide application ranges (secondary batteries, gas sensors, etc.) are expected.