In a light sensor test unit, a test circuit is built in a display panel and connected to an output node of a light sensor which senses an intensity of an external light. When external light having a predetermined intensity is provided to the light sensor, the test circuit outputs a driving signal in response to a sensing signal output from the output node. A test pixel part includes pixels selected from a plurality of pixels arranged in the display panel and receives the driving signal from the test circuit to display a gray-scale corresponding to the driving signal. A brightness measurer measures a brightness corresponding to the gray-scale displayed in the test pixel part to compare the measured brightness with a predetermined brightness, thereby testing whether the light sensor built in the display panel is normally operated.