To solve balance problem between flexibility and energy density for graphene-based fiber electrodes, graphene oxide (GO) with nanosheet size larger than 20 mu m and Mn3O4 nanocrystals with a size of about 5 nm were used as assembled units, reduced graphene oxide (RGO)/Mn3O4 hybrid fibers with excellent flexibility and high capacitance were prepared by a simple and scalable wet-spinning approach and followed by treating in a solution of hydrazine hydrate, in which GO could be reduced into RGO, while Mn3O4 nanocrystals were maintained after hydrazine vapor reduction treatment. A suitable amount of Mn3O4 nanocrystals not only maintained the tensile strength and flexibility of the obtained fiber electrode, but also obviously improved its capacitance. RGO/Mn3O4-30 fiber electrode showed a maximum volumetric capacitance of 311 F cm (3) at a current density of 300 mA cm (3) on the basis of maintaining the electrode good flexibility and mechanical strength. By intertwining two as-prepared RGO/Mn3O4-30 flexible hybrid fiber electrodes, flexible symmetric all-solid-state fiber supercapacitor was assembled by using PVA/H3PO4 as the gel electrolyte. The assembled device showed a maximum volumetric capacitance of 45.5 F cm (3) at a current density of 50 mA cm (3), and remarkable flexibility and mechanical property to tolerate long-term and repeated bending. Moreover, the assembled device exhibited a maximum volumetric energy density of 4.05 mWh cm (3) and a maximum volumetric power density of 268 mW cm (3). When three flexible symmetric all-solid-state fiber supercapacitors were connected in series, the series device could be used to power a red light emission diode (LED) lamp after being fully charged, demonstrating its potential application as an efficient energy storage component for flexible electronics. (C) 2017 Elsevier Ltd. All rights reserved.