Disclosed herein are a liquid crystal display device including a photosensor for sensing brightness of external light such that the photosensor can stably sense the external light without influence of temperature or external noise, and a method for automatically controlling the brightness, the liquid crystal display device includes a liquid crystal panel having a display area and a non-display area defined therein and including first and second substrates facing each other and a liquid crystal layer filled between the first and second substrates, the first substrate having an outer portion protruding from the second substrate, a gate driver and a data driver formed in the non-display area of the first substrate, a photosensor formed in the outer portion of the first substrate, a dummy sensor formed in the non-display area of the first substrate covered by the second substrate, a backlight unit formed below the liquid crystal panel, and a control unit including a timing controller for controlling the gate driver, the data driver and the backlight unit, and a power supply unit for supplying a power voltage.