ZnO thin films were deposited on corning glass substrates by RF magnetron sputtering at room temperature. The dependence of crystal structure, morphology and optical properties on post-deposition annealing was investigated using XRD, AFM and UV–vis Spectrophotometer. The as-deposited films were amorphous in nature. Post-deposition annealing for 1h in air caused an improvement in crystallinity with preferential orientation along (002) direction. Better crystallinity was observed at 673K. Crystallite size increased from 16 to 27nm as annealing temperature was increased from 473 to 873K. It is evident from AFM analysis that rms roughness of the film annealed at 673K is minimum indicating better optical quality. Optical measurements show a decrease in direct band gap from 3.289 (as deposited) to 3.25eV when the annealing temperature was increased to 673K. With further increase in temperature, band gap increases.