An over-driving device is provided. In a first frame, a compression circuit compresses a first image signal to generate a first compression image signal, and a buffer temporarily stores the first compression image signal. In a following second frame, the compression circuit compresses a second image signal to generate a second compression image signal, and the buffer outputs the first compression image signal to serve as a first buffer image signal. A comparison circuit compares the second compression image signal and the first buffer image signal and generates an enable signal according comparison result. A decompression circuit decompresses the first buffer image signal to generate a previous image signal. An over-driving unit receives the second image signal to serve to a current image signal and receives the previous image signal and the enable signal. The over-driving unit over drives the display device or not according to the enable signal.