A method of manufacturing a flexible substrate is provided. The method includes coating a precursor including an inorganic polymer on the flexible substrate, curing the precursor including the inorganic polymer, and oxidizing a surface of the cured precursor including the inorganic polymer to form an oxide layer. Accordingly, an organic/inorganic barrier layer may be formed by only one coating process of a thin film. In this case, oxygen plasma or infrared ray/ozone processing may be performed at atmospheric pressure, thereby reducing process costs and equipment costs by not using vacuum equipment.