Disclosed herein is a method of manufacturing a flange for wind towers through a ring rolling process. The ring rolling process makes use of a ring rolling machine. The ring rolling machine includes a main roll which presses a circumferential outer surface of a blank, a pressure roll which presses a circumferential inner surface of the blank, and a pair of axial rolls which press upper and lower surfaces of the blank. The method includes expanding an inner diameter and outer diameter of the blank using the ring rolling machine, transferring the pressure roll vertically so that a protrusion provided on a circumferential outer surface of the pressure roll comes into contact with the circumferential inner surface of the blank, and forming a depression in the circumferential inner surface of the blank using the protrusion of the pressure roll.