We investigated visible-light-induced photocatalytic activities of bismuth tungstate (Bi₂WO₆) polycrystalline particles with hierarchical "flake-ball" architecture. The flake-ball particles, which are assemblies of polycrystalline flakes composed of rectangular plate-like crystallites, were prepared by a facile hydrothermal reaction of bismuth nitrate and sodium tungstate solution at 433 K for 20 h. Figure shows FE-SEM images of the flake-ball particles. The diameter of the spherical particles was 3-4 micrometers. The lateral size and the thickness of the rectangular plate-like crystallites in flakes were a few hundreds of nanometers and 20-35 nm, respectively. The flake-ball particles exhibited the highest photocatalytic activity among a series of Bi₂WO₆ prepared by hydrothermal reactions. It is suggested that the high photocatalytic activity is due to not only the large surface area but also the high crystallinity. The flake-ball particles promote photocatalytic oxidative decomposition of acetic acid and gaseous acetaldehyde into carbon dioxide under visible light irradiation. The photocatalytic activities were higher than those of nitrogen-doped titanium dioxide as a visible-light-sensitive photocatalyst. Calcination at 873 K in air enhanced the visible-light-induced activity for oxidative decomposition of acetic acid. The ball-like morphology was remained after calcination, although the BET specific surface area was decreased. The flake-ball particles would be beneficial for practical use in photocatalytic water-purification systems because of the visible-light-induced photocatalytic activity for oxidative decomposition and the feasible separation of photocatalyst powders from aqueous solution by sedimentation and filtration.