A display apparatus and method for adjusting the gray level depending on environment illumination is used to adjust the average-gray-level of an image frame with a plurality of pixels. The display device includes a display-panel, an optical-sensor, a controller, and an adjusting module. The display-panel displays the image frame with the plurality of pixels. The optical-sensor is used to obtain the environment illumination. The controller is used to drive the display-panel to display the image frame. The adjusting module is used to adjust gray-levels of the pixels according to the environment illumination. When the environment illumination is too high, the adjusting module raises the gray-level of partial or all pixels; and when the environment illumination is too low, the adjusting module reduces the gray-level of partial or all pixels; so that visual effect of the display device is enhanced.