The present invention is to provide a method of polishing a glass substrate required to have extremely high surface smoothness and surface accuracy like glass substrates for mask blanks. The invention relates to a method of polishing a glass substrate which comprises polishing the glass substrate with a polishing pad while supplying a polishing slurry comprising an abrasive material and water to the polishing pad, wherein the polishing slurry contains at least one member selected from the group consisting of pullulan and water-soluble alcohols which are polyvalent organic compounds having two or more OH groups. The slurry preferably has a pH adjusted to 0.5-4.