Morphology evolution of anatase TiO2 nanocrystals under a hydrothermal condition (pH = 9.5) was observed and a relationship between the morphology evolution and photo-catalytic activity was investigated. A titanium hydroxide nanogel was neutrally precipitated from a TiCl4 aqueous solution and then used as the precursor in the hydrothermal process. In the hydrothermal process, the nanogel was finally crystallized and grew to a capped bipyramidal nanocrystal which has faceted {101} pyramidal faces and are capped with rounded (001) faces, passing through an elongated structure with zigzag {101} faces. The final morphology was evolved via four growth stages in series: (i) formation and growth of anatase nuclei with consuming the nanogel, (ii) rapid growth along [0 0 1] direction by oriented attachments between (0 0 1) faces to produce an elongated structure with zigzag {101} pyramidal faces, (iii) flattening of zigzag {101} pyramidal faces by solution and precipitation, and (iv) rapid growth along [0 0 1] direction by solution and precipitation (Ostwald ripening). The photo-catalytic activity of anatase nanocrystals was closely related to the morphology evolution. Especially, the capped bipyramidal nanocrystals in the last growth stage showed an excellent photo-catalysis behavior: some of them were more than three times active than a commercial photo-catalyst (P25). The ultra-high photo-catalytic activity originated in the well-developed {101} surfaces, since the 4-coordinated Ti ions in the step edge of {101} face effectively adsorbed hydroxyl ions. Therefore, the particle morphology (surface property) was important as much as the particle diameter in TiO2 photo-catalysis. (c) 2005 Elsevier B.V. All rights reserved.