Development of mechanically flexible carbon nanofibers is highly desired for the applications in modern flexible electronics and energy storage devices. This work reports the manufacture and characterization of highly flexible carbon nanofibers (CNFs) and graphitic carbon nanofibers (GCNFs) from a non-processable heteroaromatic rigid-rod polymer, polybisbenzimidazobenzophenanthroline-dione (BBB). The flexible CNFs/GCNFs were prepared by a newly developed method of electrospun nanofiber template solid-state polymerization, followed by carbonization/graphitization. In specific, BBB nanofibers were prepared first by the nanofiber template solid-state polymerization method using polyimide as template by electrospinning and heat treatment (500 A degrees C). Subsequently, CNFs/GCNFs were obtained through carbonization under different temperatures of 1200-2700 A degrees C. SEM, HRTEM, Raman spectroscopy, and XRD were used to characterize the morphologies and microstructures. Intriguingly, the BBB-derived CNFs/GCNFs presented extremely good mechanical flexibility that resisting to readily bending, folding, and kneading. Hence, this newly developed extremely flexible BBB-derived CNFs/GCNFs with such good performance could have great potential applications such as using as electrode materials for flexible electrochemical devices.