Designing a high-performance electrocatalyst is very important for producing energy systems such as glucose-oxygen fuel cells (GFCs). Here, we report the preparation of a nanocomposite material consisting of different weight percentages of reduced graphene oxide-supported (5, 10, and 20 wt %) cobalt oxide nanoparticles (rGO-Co3O4). The corresponding modified electrodes exhibited bifunctional electrocatalytic behavior toward both glucose oxidation and oxygen reduction reaction (ORR). The rGO-Co3O4 nanocomposite material is prepared and characterized by thermogravimetry analysis, X-ray diffraction, diffuse reflectance spectroscopy, Fourier transform infrared spectroscopy, selected area electron diffraction, the Brunauer-Emmett-Teller method, the Barret-Joyner-Halenda method, energy-dispersive X-ray spectroscopy, high-resolution transmission electron microscopy, and electrochemical analysis. Among different weight percentages of rGO, 10 wt % GO in the rGO-Co3O4 nanocomposite-modified electrode shows stable electrooxidation of glucose with a rapid response time of 1 s with a 0.4 mu M limit of detection and a 1008 mu A mM(-1) cm(-2) sensitivity. In addition, rGO-Co3O4 (10 wt % GO)-modified electrodes show less negative potential with a large kinetic current and better catalytic durability for ORR. Thus, rGO-Co3O4 (10 wt % GO) is employed as a bifunctional catalyst in cost-effective GFCs and obtains a power density output of 0.7319 mW cm(-2).