An optical fiber is prepared by applying a liquid electron beam-curable resin composition to a bare optical fiber or a coated optical fiber having a primary or secondary coating on a bare optical fiber, irradiating electron beams to the resin composition on the optical fiber for curing while the optical fiber passes a zone under substantially atmospheric pressure, and providing a magnetic field and optionally an electric field in the zone for thereby improving the efficiency of electron irradiation. The method can comply with the increased drawing speed of the bare optical fiber and does not detract from the transmission properties of the optical fiber.