In this paper, we present a new method for synthesis of water-soluble Co3O4 nanocrystals (NCs) by microwave irradiation with controllable temperature. Spherical or cubic Co3O4 NCs (10-20 nm) were synthesized rapidly within 10 min at 100-140 degrees C. MPA was used as stabilizer to synthesis Co3O4 NCs in this paper, which leads to the controlled growth and narrow sizes distribution of the NCs. Our results demonstrated that the shape of Co3O4 NCs was reaction temperature-dependent, i.e., low temperature is in favor of the synthesis of spherical NCs, and high temperature is in that of cubic NCs. (C) 2006 Elsevier Ltd. All rights reserved.