A device and a method for controlling brightness of an OLED display device are disclosed.The method for controlling brightness of an OLED display device includes the steps of forwarding external brightness control information in a PWM signal or a brightness control data, selecting and normalizing either the PWM signal or the brightness control data into an external brightness adjusting gain, analyzing a received video data to detect a peak brightness value, multiplying the peak brightness value by the external brightness adjusting gain to produce a final peak brightness value, adjusting the R/G/B maximum gamma voltage values according to the final peak brightness value, and generating R/G/B reference gamma voltage sets by using the R/G/B maximum gamma voltage values adjusted thus.