TiO(2) nanoparticles have been widely utilized in photocatalysis, but the atomic level understanding on their working mechanism falls much short of expectations. In particular, the correlation between the particle structure and the photocatalytic activity is not established yet, although it was observed that the activity is sensitive to the particle size and shape. This work, by investigating a series of TiO(2) anatase nanoparticles with different size and shape as the photocatalyst for water oxidation, correlates quantitatively the particle size and shape with the photocatalytic activity of the oxygen evolution reaction (OER). Extensive density functional theory (DFT) calculations combined with the periodic continuum solvation model have been utilized to compute the electronic structure of nanoparticles in aqueous solution and provide the reaction energetics for the key elementary reaction. We demonstrate that the equilibrium shape of nanoparticle is sensitive to its size from I to 30 nm, and the sharp-crystals possess much higher activity than the flat crystals in OER, which in combination lead to the morphology dependence of photocatalytic activity. The conventionally regarded quantum size effect is excluded as the major cause. The physical origin for the shape activity relationship is identified to be the unique spatial separation/localization of the frontier orbitals in the sharp nanoparticles, which benefits the adsorption of the key reaction intermediate (i.e., OH) in OER on the exposed five-coordinated Ti of {101} facet. The theoretical results here provide a firm basis for maximizing photocatalytic activity via nanostructure engineering and are also of significance for understanding photocatalysis on nanomaterials in general.