Ultraviolet radiation process applies to manufacture semiconductor devices in order to enhance the thermal stability of the photoresist film of semiconductors wafers. A method, in ultraviolet radiation process, enabling effective treatment of the photoresist employing ultraviolet irradiation by preventing the deformation of the photoresist which is caused by exposing it to high ultraviolet radiation at the beginning of exposure. This method employs ultraviolet irradiation, in which the photoresist is exposed to ultraviolet radiation of low intensity at the beginning of exposure, and then exposed to ultraviolet radiation,the intensity of which increases little by little or in steps.