In this paper, we report temperature dependent studies of ZnO nanocrystallites deposited by the sol-gel spin coating process. The films are annealed at different temperature from 500 to 900 degrees C to grow the size of crystallites in controlled environment. The crystallite size, lattice parameters, and strain in the nanocrystallites are calculated. A coarsening kinetics is understood by Lifshitz, Slyozov and Wagner (LSW) theory and the activation energy of the coarsening process is estimated. Optical absorption and Photoluminescence (PL) measurements were carried out to investigate the effect of coarsening on optical properties of the ZnO thin films. The origin of PL spectra is explained in terms of the stabilization of structural defects and size of the crystallites. (C) 2010 Elsevier B.V. All rights reserved.