A method for reducing the tilt of an optical unit during manufacture of an image sensor includes the steps of: providing a semimanufacture of the image sensor, carrying out a preheating process, carrying out an adhesive application process, carrying out an optical unit mounting process, and carrying out a packaging process. Due to the preheating process, the semimanufacture will be subjected to a stabilized process environment during the adhesive application process and the optical unit mounting process, so as for the optical unit to remain highly flat once attached to the semimanufacture. The method reduces the chances of tilt and crack of the optical unit and thereby contributes to a high yield rate.