Porous Co3O4 nanowires with large aspect ratio have been obtained by annealing long Co(CO3)0.5(OH)·0.11H2O precursor nanowires synthesized by a facile hydrothermal method. The results show that the amount of the additive (urea) has an important impact on the morphology of the as-synthesized cobalt-carbonate-hydroxide intermediate, where the uniformity and the overall structure can be controlled by changing the urea concentration. After the heat treatment, the as-obtained phase-pure Co3O4 nanowires with a well retained structure are applied as the electrode material for supercapacitors, and the sample exhibits excellent performance with a high specific capacitance of 240 F g−1 after 2000 charge/discharge cycles, corresponding to a retention of 98% of the initial capacitance.