Abstract: Super-hydrophobic SiO2 thin films were prepared by sol-gel processing，a phase separation and self-assembly technique. Tetraethylorthosilicate (TEOS) was taken as the resource of silica and polyacrylic acid (PAA) was added to induce phase separation. The mircrostructure evolution of the silica films were controlled by phase separation which was enhanced properly by the addition of PAA. The effect of PAA agent on mircrostructure and the contact angle of SiO2 films were investigated. The surface of thin films was characterized with SEM. The results showed that the surface roughness of SiO2 thin film increased with the addition of PAA. After the films were modified by chemical vapor deposition of trimethylchlorosilane (TMCS)，a self-assembled monolayer of trimethylchlorosilane was formed on the films and SiO2 thin films with super-hydrophobic property with water a contact angle as high as 158° were obtained.