ITO thin films deposited on glass substrate with RF magnetron sputtering were vacuum annealed at 100, 200 and $300^{circ}C$ for 30 minutes and then effect of annealing temperature on the structural, electrical and optical properties of ITO films were investigated. The structural properties are strongly related to annealing temperature. The annealed films above $100^{circ}C$ are grown as a hexagonal wurtzite phase and the largest grain size is observed in the films annealed at $300^{circ}C$. The electrical resistivity also decreases as low as $4.65{ imes}10^{-4}{Omega}cm$ with a increase in annealing temperature and ITO film annealed at $300^{circ}C$ shows the lowest sheet resistance of $43.6{Omega}/{Box}$. The optical transmittance in a visible wavelength region also depends on the annealing temperature. The films annealed at $300^{circ}C$ show higher transmittance of 80.6% than those of the films prepared in this study.