Calcium tungstate (CaWO4) nanophosphors with the particle sizes from 35 to 90 nm were synthesized by a hydrothermal process through exactly controlling the pre-treated conditions. The influence of particle size on oxygen vacancy and photoluminescence properties in CaWO4 nanophosphors was investigated and discussed. The crystal structure of the CaWO4 nanophosphors presented a certain level of distortion due to the high concentration of oxygen vacancy. Under 350 nm excitation, a clear green emission aroused by oxygen vacancy was observed. The possible luminescence processes for the matrix and oxygen vacancy were proposed. The luminescence spectra of the nanophosphors revealed that the emission and absorption intensity aroused by oxygen vacancy were both enhanced when the size is decreased. On the basis of the above results, the essential relationship between particle size and oxygen vacancy in CaWO4 nanophosphors was concluded that the concentration of oxygen vacancy could be increased by reducing its size, which was further confirmed by decay lifetimes. (C) 2013 Elsevier Ltd. All rights reserved.