Co3O4 nanosheets with superior electrochemical properties are prepared by a spray drying process. The precursor powders with a hollow and a thin-walled structure, obtained by the spray drying process, turn into nanosheets after a post-treatment process at temperatures below 600°C. Citric acid dissolved in the spray solution of the cobalt precursor is a key material for the formation of the hollow thin-walled precursor powders. The mean crystallite sizes of the Co3O4 nanosheets post-treated at 200 and 400°C are 17 and 24nm, respectively. The mean powder sizes consisting of the nanosheets, as measured from the TEM images, are similar to the mean crystallite sizes of the powders. The discharge capacities of the Co3O4 nanosheets post-treated at 200 and 400oC are 925 and 1102mA h g-1, respectively, at a current density of 1400mA g-1 after 100 cycles, and the corresponding capacity retentions, measured after the first cycle, are 91% and 110%. The initial discharge capacities of the Co3O4 nanosheets post-treated at 400°C are 1199 and 1122mA h g-1 at high current densities of 3500 and 7000mA g-1, respectively, and the discharge capacities after 100 cycles are 868 and 695mA h g-1.