Towards rapid development of lightweight, flexible, and even wearable electronics, a highly efficient energy-storage device is required for their energy supply management. Graphene fiber-based super capacitor is considered as one of the promising candidates because of the remarkable mechanical and electrical properties of graphene fibers. However, supercapacitors based on bare graphene fibers generally suffer a low capacitance, which certainly restricts their potentially wide applications. In this work, hierarchically structured MnO2 nanowire/graphene hybrid fibers are fabricated through a simple, scalable wet-spinning method. The hybrid fibers form mesoporous structure with large specific surface area of 139.9 m(2) g(-1). The mass loading of MnO2 can be as high as 40 wt%. Due to the synergistic effect between MnO2 nanowires and graphene, the main pseudocapacitance of MnO2 and the electric double layer capacitance of graphene are improved simultaneously. In view of the practical demonstration, a highly flexible solid-state supercapacitor is fabricated by twisting of two MnO2/graphene fibers coated by polyvinyl alcohol/H3PO4 electrolyte. The supercapacitor exhibits a high volumetric capacitance (66.1 F cm(-3), normalized by the total volume of two fiber electrodes), excellent cycling stability (96% capacitance retention over 10,000 cycles), high energy and power density (5.8 mWh cm(-3) and 0.51 W cm(-3), respectively). (C) 2015 Elsevier B.V. All rights reserved.