Abstract: In the paper preparation and research of hydrophobic optical SiO 2 thin film are reported. The solution is prepared with a sol gel process and then modified by TMCS(trimethylchlorosilane). The films are prepared by dip coating method and different post methods are used to treat them. Ellipsometry, FTIR and contact angle instrument are used to measure the physical properties of the thin films. The results show that the refractive index of thin films are variable in the range from 1.35 to 1.20, OH groups on the surface of SiO 2 particles are replaced by CH 3 groups, the contact angle is increased to 137 degree of modified films, which are treated with mixed ammonia and water, from 40 degree of unmodified films.