A large number of hierarchical Co3O4 nanostructures have been prepared via the thermal treatment of an urchinlike cobalt-hydroxide-carbonate precursor, which was obtained by simple hydrothermal treatment of CoCl2 solution in the presence of urea and K2SO4. The as-obtained cobalt-hydroxide-carbonate precursor has been characterized by XRD, SEM, TEM, XPS, IR and TGA. To study the formation process of this urchinlike precursor, the influences of experimental parameters, such as reaction time and inorganic salts, etc., on the morphology have been investigated during the synthesis. It was found that the morphology of cobalt-hydroxide-carbonate was strongly dependent on the sulfate. Calcination of the precursor at 350 degrees C led to the formation of well-defined hierarchical Co3O4 nanostructures with a surface area of 60 m2 g(-1). The electrochemical properties of Co3O4 nanostructures were also investigated.