Abstract: Mixtures of polyethyleneimine (PEI) and ethylene tar pitch (0, 15, 20, 30 wt% of PEI) were melt-spun, stabilized, carbonized and activated to prepare nitrogen-doped (N-doped) activated carbon fibers (ACFs). The morphology, porous structure and surface chemistry of the N-doped ACFs were characterized by N₂ adsorption, XPS and SEM. Their electrochemical performance as supercapacitor electrode materials was investigated. Results indicate that the specific surface area, pore volume and number of nitrogen-containing functional groups of the N-doped ACFs are much increased compared to undoped ACFs. PEI pyrolysis during the carbonization of the fibers leads to the formation of hollow N-doped ACFs that increases the utilization% of the surface area, resulting in a significant increase of the specific capacitance. When 20 wt% PEI was added, the specific surface area of the N-doped ACF reached 2 756 m²/g, its pore sizes ranged from 0.7 to 2 nm, and its specific capacitance reached 314 F/g at 0.5 A/g, which is much higher than that (194 F/g) of the undoped ACF.