A method for assembling a lens module with an image sensor, includes providing a holder having first and second receiving spaces, disposing the image sensor in the second receiving space, locating lens barrel at a first position in the first receiving space, casting a light beam on the lens module to allow the lens module to converge the light beam into a light spot on the image sensor, determining if the light spot is at its smallest to determine if a distance between the lens module at the first position and the image sensor is equal to a focal length of the lens module, moving the lens module to a second position in the first receiving space to achieve a predetermined distance from the first position, and positioning the lens module at the second position.