The invention discloses a noise reduction device for adjusting gray levels of a video sequence to reduce noises. The noise reduction device of the invention includes a motion detector and a first noise reduction module. The motion detector is used for determining whether an inputted pixel is a motion pixel or a static pixel. If the pixel is a motion pixel, the first noise reduction module directly outputs the pixel without adjustment. If the pixel is a static pixel, the first noise reduction module adjusts the gray level of the pixel according to previous image frame. Because the invention only stores previous image frame, and does not utilizes multiple continuous image frames to adjust the present image frame. Therefore, the display system of the invention needs only to be installed with one buffer, and hardware resource can be further saved.