In this article, we report the preparation and thermal properties of polyimide-mesophase pitch (MP) composite nanofibers and associated nanofiber nonwoven mats produced using an electrospinning process. The addition of MP increased the thermal conductivities of both the individual composite nanofibers and the in-plane conductivities of the nanofiber mats. The out-of-plane conductivity of the mats remained relatively low due to low through thickness connectivity between the nanofibers. These nanofiber mats are flexible and very thin and are good candidates for thermal management films for future flexible electronic devices. POLYM. ENG. SCI., 54:977-983, 2014. (c) 2013 Society of Plastics Engineers