High quality pure YAlO3 crystal with dimension of Phi 30 x 50 mm(2) was grown by Czochralski technique. UV irradiation and air annealing bring additional absorption in the region 200-800 nm. The absorption spectra of perfect YAlO3 and YAlO3 containing cation vacancy (aluminium vacancy and yttrium vacancy) were calculated using density functional theory code CASTEP. Comparison of the simulated absorption spectra with the experimental absorption spectra of YAlO3 after UV irradiation and air annealing treatments shows that cation vacancies are responsible for part of the coloration on YAlO3 Crystal. (C) 2009 Elsevier B.V. All rights reserved.