Co3O4 and Co3O4/CoO nanoparticles have been synthesized by a one-step solution combustion process by adjusting the molar ratio of citric acid (fuel) and Co(NO3)2·6H2O (oxidizer). The effects of citric acid/Co(NO3)2·6H2O molar ratios on phase composition and morphology of products were investigated by XRD and SEM. With the increase of the fuel dosage, the products transformed from granular aggregates of cubic Co3O4 into a mixture of cubic Co3O4 and tetragonal CoO with fluffy sheet morphologies. Electrochemical measurements indicated that the products (Co3O4) showed a capacitance up to 179.7 F·g−1 (at 0.2 A·g−1) when the citric acid/Co(NO3)2·6H2O molar ratio was 7/27. Significantly, the capacitance could be further improved by 102% (362.8 F·g−1 at 0.2 A·g−1) after annealing at 350°C for 3h under nitrogen atmosphere. This annealed sample also demonstrated decent rate performance (285.7 F·g−1 at 4 A·g−1) and cycling stability (73.5% retention after 1000 cycles). The current study suggests that this process has promise in large-scale production of electrode materials for supercapacitors.