In the present work, we report the enhanced electrochemical performance of ball milled CoO nanoparticles for supercapacitor applications. The mechanical ball milling provides clean physical processes to prepare nanoparticles from CoO micropowders for excellent electrochemical performances. The performances of CoO samples at different milling times have been researched. With the increase of milling time the specific capacitance of CoO samples increases. The average size of CoO nanoparticles which have been milled for 96 h is estimated to be 5–20 nm by Transmission Electron Microscopy (TEM) analysis showing clear edges having superior boundary crystallinity. This clear edge superior boundary crystalline shape favours rapid electron and ion transport. The electrochemical behaviour is analyzed in a three electrode system using 1 M KOH solution as the electrolyte in terms of cyclic voltammetry, cyclic charge–discharge, and electrochemical impedance spectra. The CoO nanoparticle electrode exhibits a specific capacitance of 600 F g⁻¹ at 0.5 A g⁻¹ constant discharge current density. The high specific capacitance and the stability of the CoO nanoparticle electrode are attributed to good crystallinity and large specific surface area. The specific capacity retention is 96.6% at a current density of 2 A g⁻¹ and 95.3% at a current density of 4 A g⁻¹ over 2000 charge–discharge cycles. The excellent cyclic stability indicates that nanocrystalline CoO is an excellent supercapacitor electrode material.