The effect of annealing on the physical properties of DC and RF sputtered ITO thin films has been investigated. Two series of samples were deposited onto glass substrates, the first one consists of several In2O3:Sn (ITO) prepared by DC reactive sputtering with different partial pressure of oxygen (ppo); in the second one, the ITO samples were done by RF sputtering with different RF powers. Annealing experiments were done in various conditions: in air, in vacuum and in Ar atmosphere for temperatures in the 100 °C to 500 °C range. Structural properties were studied using Scanning Electron Microscopy (SEM), the usual X-ray diffraction (XRD) and also in-situ X-ray diffraction where X-ray spectra were recorded while the sample was being annealed. The optical properties were inferred by means of a spectrophotometer with wavelength in the 200–900 nm range. It was found that, for DC sputtered films, annealing favours the existing texture either $\langle 111\rangle$ or $\langle 100\rangle$; while, for the RF sputtered ones it always favours the $\langle 111\rangle$ texture. Annealing induced some similar changes in both series of samples such as the increase of grain size, the decrease of the strain and the electrical resistivity. The values of the optical transmission decreases after annealing at temperatures T = 400 °C and T = 500 °C for the DC and the RF sputtered films respectively. The decrease seems to be more important in the DC than in the RF sputtered ITO films.