Flake-like single-crystalline ZnO nanocrystals with porous structure have been achieved, in which precursor of Zn4CO3(OH)(6)center dot H2O was first prepared by mild hydrothermal method with urea as the homogeneous precipitant and decomposed into small ZnO nanocrystals after being calcined at 400 degrees C, then the small ZnO nanocrystals self-assemble to form flake-like ZnO aggregates. The ZnO nanoflakes have lateral dimensions up to micrometer with the plane normal to [0 0 1] direction. The UV-vis absorption reveals that the ZnO nanoflakes have strong absorption in the UV region. The advantages of our method for the synthesis lie in the low temperature and mild reaction condition, which permit large-scale production at low cost. (C) 2005 Elsevier Ltd. All rights reserved.