Two methods were utilized to prepare hydrophobic silica AR coatings by sol-gel progress.With hexamethydisizane (HMDS) as tri-methyl modifiers, methyl-modified silica sols was prepared and hydrophobic coatings were achieved by depositing such modified sols on substrate.Another way was a post-treatment method.By treating the standard TEOS-based sol-gel silica AR coatings with select vapor produced hydrophobic coatings too.And the resulting sol-gel AR coatings were tested for their stability in vacuum.As a result of the comparative study, vapour-phase silylation was more suitable to prepare hydrophobic anti-reflective coatings with required high optical performance.