The influence of the particle size of an active material on its performance as a supercapacitor electrode was reported. Nickel oxide nanoparticles (NiO NPs) with a uniform particle size were synthesized via a facile sol–gel method, and various sizes of NiO NPs (8, 12, and 22 nm) were achieved by calcination at various temperatures (300, 400, and 500 °C). TEM observations and XRD analysis were used to determine the particle size of the NiO NPs. The field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM) images showed flake-like morphologies, which consisted of interconnected nanoparticles with a porous channel to facilitate the diffusion of the electrolyte. The NiO NPs with an average particle size of 8 nm gave the highest specific capacitance value of 549 F g⁻¹ at a scan rate of 1 mV s⁻¹ compared to the NiO NPs with average particle sizes of 12 and 22 nm. These results suggest that the particle size of the NiO nanostructure plays an important role because of the presence of a higher number of active sites for a faradaic reaction.