Abstract: A brightness adjusting device for a light-emitting diode (LED) display panel has at least one photo sensor, a control circuit, a timing controller and a driving circuit. The LED display panel is composed of multiple LEDs being divided into groups according to color. The photo sensor sequentially senses brightness of the groups of LEDs and generates brightness values. The control circuit compares the brightness values to standard brightness values. Based on the compared results, the driving circuit adjusts the brightness of corresponding LEDs.