The hexagonal WO3 hierarchical nanostructures assembled by WO3 nanorods are synthesized by a seed-assisted hydrothermal process. The SEM and TEM images indicate that the assembled WO3 nanostructures with seeds could be controlled by modulating the precursor concentration, to form 6-fold-symmetrical shapes of WO3, irregularly 6-fold flowerlike structures of WO3 and irregular WO3 aggregation. On the basis of the experimental results of products obtained with and without seeds, we propose that the reduced activation energy barrier of WO3 nucleation and the 6-fold symmetry intrinsically of WO3 crystallites trigger the hexagonal shape. Compared with the original WO3 flocky spheres prepared without seeds, hexagonal WO3 hierarchical nanostructures prepared with seeds possessed superior photochromic properties. And the 6-fold-symmetrical WO3 hierarchical nanostructures obtained at a lower precursor concentration got the optimized photochromic properties. (C) 2013 Elsevier Ltd. All rights reserved.