The size effect of the optical absorption in one of the most typical organic nanocrystals, alpha-perylene, was studied. The true absorption spectra of nanocrystals suspended in water were measured within an integrating sphere by collecting the whole scattered light. It was experimentally confirmed that the absorption spectra show blueshift as the crystal size is decreased. This was reproduced by the size-dependent absorption spectra calculated for spherical nanocrystals with isotropic and anisotropic dielectric constants to elucidate three mechanisms of the blueshift: size-dependent change in the shape anisotropy of the crystals, longitudinal-transverse mixing, and light propagation effects including scattering on absorption. (C) 2014 Elsevier B.V. All rights reserved.