Chemical treatment using concentrated nitric acid (16 M) not only induced significant improvement of mechanical and electrical properties of carbon nanotube fibers due to the enhanced interfacial interaction but also allowed much more efficient deposition of polyaniline for developing fiber-shaped supercapacitors. After the 2 h treatment, the acidized fiber had a tensile strength of 1.52 GPa and an electrical conductivity of 1050 S cm(-1), increased by 52% and 128%, respectively, compared with the untreated one. By depositing polyaniline for 10 min around the fiber, the composite fiber had a volumetric capacitance of 239 F cm(-3), 17% higher than that without the acid treatment. For a long time treatment up to 6 h, although the strength and conductivity decreased slightly, the composite fiber had a super high volumetric capacitance up to 299 F cm(-3). The improvement of electrochemical performance is attributed to the increased deposition rate and structural change of polyaniline due to the existence of functional groups on the fiber surface.