A series of sulfonated polyimide (SPI) composite proton exchange membranes with different contents of an ionic liquid-functionalized graphene oxide (FGO) were prepared in order to study the effects of FGO fillers on the membranes properties. Graphene oxide (GO) was synthesized by the modified Hummers method and then was functionalized with 1-methylimidazole. The phosphoric acid-doped FGO nanosheets were incorporated into the sulfonated polyimide for preparation of composite proton exchange membranes. The sulfonated polyimide (SPI) was synthesized via a solution imidization method. A series of techniques such as FT-IR, H-1-NMR, XRD and XPS were used for the characterization of synthesized materials. The membranes properties such as thermal and mechanical stability, morphology, water uptake, phosphoric acid uptake and proton conductivity were investigated. The proton conductivities of the membranes were measured as a function of temperature in ambient humidity and also 80% relative humidity (RH) conditions. The pure SPI showed its maximum proton conductivities of 0.0282 and 0.0864 S cm(-1) at ambient and 80% RH, respectively, while SPI/FGO 5% showed the maximum proton conductivity values of 0.0772 5 cm(-1) (at 160 degrees C and ambient humidity) and 0.1243 S cm(-1) (at 120 degrees C and 80% RH) among all of the samples. Copyright (C) 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.