The N incorporation into carbon nanotubes (CNTs) supporting ultrafine RuO(2) nanoparticles (NPs) has been studied. With increasing N dopant, the Raman spectrum shifts to higher wavenumbers and x-ray photoelectron spectroscopy results show the intensity ratio of graphene- to pyridine-like bonding is reduced. Cross-sectional scanning electron microscope images reveal that the uniform RuO(2) NPs are dispersed on CNTs at the N(2) flow rate of 20 sccm. Electrochemical (EC) measurements show N-doped CNTs covered with RuO(2) NPs at an N(2) flow rate of 20 sccm provide the optimal capacitive behavior with larger energy density and can be performed at the higher scan rate of 2000 mV s(-1). The distribution of RuO(2) NPs on CNT surfaces deduced from N-induced defect sites is the key point in controlling the capacitive characteristics of CNT-RuO(2) nanocomposites (NCs). The high capacitance is due to the well-dispersed RuO(2) particles on CNTs incorporated with N atoms. Such NCs are promising for energy storage devices with high EC efficiency.