Abstract: A method for adjusting brightness of a display device acquires optical signals from a plurality of light sensors positioned on the display device. The optical signals are converted into digital signals. An ambient brightness value is determined according to the digital signals. If the ambient brightness value is not less than a predetermined value, a calibration brightness value of the display device is calculated according to the ambient brightness value. If the calibration brightness value is equal to zero and the display device has received video signals during a predetermined time interval, a current brightness value of the display device is maintained. If the calibration brightness value is not equal to zero and the display device has received the video signals during the predetermined time interval, the current brightness value of the display device is adjusted according to the calibration brightness value.