To study mechanical and structural stabilities of polyimide (PI) and carbon filler composites by Joule heat, PI-vapor grown carbon fiber (VGCF) composites were fabricated by in situ polymerization to realize excellent dispersion of VGCFs in PI matrix. Adoption of VGCFs similar to rigid carbon fibers is the strategy to investigate the conductive mechanism theoretically in terms of thermal fluctuation-induced tunneling conduction. Dynamic tensile modulus and X-ray intensity under applied electric field were measured to investigate frequency-temperature dependence of the modulus by Joule heat in relation to the electron transfer mechanisms. Such simultaneous measurements for the mechanical and structural properties under electric field provided important information about the characteristic of polymer-filler composites. As the results, good heat resistance of PI-VGCF composites was clarified to be attributed to very few thermal fluctuation of PI chain arrangement in spite of collision between electrons flowed out from the VGCF gaps and atoms of PI chains. (C) 2014 Elsevier Ltd. All rights reserved.