The ultrafine Co₃O₄ nanoparticles are successfully prepared by a novel solvothermal–precipitation approach which exploits the supernatant liquid of Co₃O₄ nanoflake micropheres synthesized by solvothermal method before. Interestingly, the water is only employed to obtain the ultrafine nanoparticles in supernatant liquid which was usually thrown away before. The microstructure measurement results of the as-grown samples present the homogeneous disperse ultrafine Co₃O₄ nanoparticles with the size of around 5–10 nm. The corresponding synthesis mechanism of the ultrafine Co₃O₄ nanoparticles is proposed. More importantly, these ultrafine Co₃O₄ nanoparticles obtained at 250 °C show the highest specific capacitance of 523.0 F g⁻¹ at 0.5 A g⁻¹, 2.6 times that of Co₃O₄ nanoflake micropheres due to the quantum size effect. Meanwhile, the sample annealed under 350 °C possesses the best cycling stability with capacitance retention of 104.9% after 1500 cycles. These results unambiguously demonstrate that this work not only provides a novel, facile, and eco-friendly approach to prepare high-performance Co₃O₄ nanoparticles electrode materials for supercapacitors but also develops a widely used method for the preparation of other materials on a large scale.