The structural and optical properties of Hg-doped TiO₂ thin films, deposited on glass and silicon substrates by sol-gel method. The films were examined in the temperature range (400 to 550), as well as at different holding times. We calculated the refractive index (n) and the optical gap from the measured transmission spectra as a function of the annealing temperature and the holding time. The calculated values ""of the index (n) and gap (Eg) are in the order of (1.63-2.5) and (3.35-2.36) respectively. The transmission coefficient varies from (50 to 95)% with annealing and holding temperature. In the case of xerogel of TiO2 doped with Hg aged for 3 months at room temperature, we observed the formation of the anatase structure, along the plane (101) in contrast to that of undoped one. Morevere, the thin films annealing between 400 and 550 C and different times, show the formation of anatase and brookite structure. The calculated grains are in order of 8.04 to 15.65 nm thin films doped as function of temperature and holding time.