Polyaniline nanofibers (PANI-NFs)/graphite oxide (GO) nanocomposites with excellent interfacial interaction and elongated fiber structure were synthesized via a facile interfacial polymerization method. This polymerization method efficiently exfoliated the expanded layer structure of GO into individual sheets and thus significantly enhanced the specific surface area. Direct evidence of a strong interaction between PANI-NFs and GO was observed from TEM results. The reduced diameter of PANI-NFs in PANI-NF/GO than that of pure PANI-NFs could shorten the diffusion distance and enhance the electro-active sites. The PANI-NFs/GO hybrid materials showed orders of magnitude enhancement in capacitance and energy density than that of individual GO and PANI-NF components. The higher PANI-NFs fraction in composites generated an energy density of 7.1 W h/kg at a power density of 80 W/kg. The enlarged electrochemical capacitance of the PANI-NFs/GO nanocomposites was attributed to the increased specific surface area, which improved the accessibility of ions from electrodes to electroactive materials as well as the improved conductivity due to the excellent interfacial interaction. PANI nanoparticles were observed in the nanocomposites when surface initiated polymerization method was used to prepare the polymer nanocomposites. At the same weight loading of PANI in the composites, fibrous PANI demonstrated higher energy density and long term stability than that of particle-shaped PANI at higher power density. (C) 2012 Elsevier Ltd. All rights reserved.