We have synthesized supercapacitor electrodes fashioned of NiO(x)/multiwalled carbon nanotubes (MWNTs), in which the controlled NiO(x) nanoparticles were prepared via a simple colloidal method and supported on the MWNTs. The sizes of the NiO(x) nanoparticles on MWNTs were systematically varied from 4 to 14 nm at a fixed metal loading of 20 wt% by changing sintering temperature in a controlled manner. The maximum specific capacitance of the NiO(x)/MWNTs was measured to be ca. 215 F per unit gram of NiO(x)/MWNTs by cyclic voltammetry (CV) in an aqueous 1 M KOH electrolyte, demonstrating that the specific capacitance of the MWNTs-supported nanosize NiO(x) is strongly dependent on the dispersion and size of the nanoparticles for the supercapacitor performance.