A method for adjusting brightness of an image, suitable for an image capturing apparatus with a flash lamp, is provided. In the method, an intensity of a main flash of the flash lamp is raised, and a sensitivity is reduced. When the main flash is fired by the flash lamp, a raw image is captured according to the reduced sensitivity for calculating a brightness mean value. A gain ratio is calculated according to the brightness mean value and a brightness target, so as to use the gain ratio to compensate the brightness of the raw image.