In a method of operating a display device supporting a variable frame mode, frame data are received during a constant active period of a frame period including the active period and a variable blank period, the received frame data are written to a frame memory in the active period, the received frame data are outputted to a data driver in the active period to display an image based on the received frame data, a time of the variable blank period is counted, and, when the time of the variable blank period reaches a predetermined threshold blank time, the frame data stored in the frame memory are outputted to the data driver in the variable blank period to display an image based on the frame data stored in the frame memory.