Device and method for controlling a luminance of a flat display minimizes a chip area, and controls a luminance of a desired portion of a display efficiently without an additional power consumption. The device includes a reference signal part for receiving an external video signal and provides a duty ratio of a scan signal and a video data desired to display. A controller converts the duty ratio from the reference signal part and generates a remaining scan time period. A display part receives and displays an image data (video data) with the application of scanning pulses having a duty ratio processed by the controller.