A multi-functional NiO/g-C3N4 (NC) hybrid nanostructure was synthesized by a hydrothermal process using melamine and Ni(OH)2 as precursors followed by thermal treatment. The optimal conditions were determined by studying the process conditions, such as the Ni(OH)2 to melamine ratio and thermal treatment temperature. The NC prepared in this study exhibited both excellent glucose sensing properties and supercapacitor properties. A very high glucose sensitivity, as high as 5,387.1 μA mM-1cm-2, and excellent energy density of 49.6Wh kg-1 at a power density of 1,064.2W kg-1 were obtained when NC was used as the electrode material for glucose sensing and symmetric supercapacitor, respectively. A flexible glucose sensing device using a flexible substrate and self-powered glucose sensor system that used the same material (NC) for the both power supply and sensing devices were also demonstrated.