The controllable synthesis of two kinds of alpha-Fe2O3 hierarchical nanostructures, i.e., three-dimentional (3D) houseleek-like and two-dimentional (2D) snowflake-like dendrites were achieved through a simple hydrothermal route by changing pH value. The growth of alpha-Fe2O3 dendrites was proceeded by self-assembly through two different modes of oriented attachment (OA): when pH <= 6, primary alpha-Fe2O3 nanoparticles attached preferentially along the six crystallographically equivalent < 1 (1) over bar 00 > directions, resulting in the formation of sixfold-symmetric dendrites. While at pH <= 5, the growth process involved two steps: firstly, primary nanoparticles aggregated to form round flakes with their up and bottom surfaces parallel to {0001} plane. These flakes stacked face-to-face with each other along [0001] direction to construct the single crystalline spindle-like alpha-Fe2O3, which then aggregated together at the tips to construct the 3D houseleek-like dendrites. As far as we know, this is the first time using different modes of OA to realize the morphology control of hierarchical structures in one reaction system. (c) 2006 Elsevier B.V. All rights reserved.