Animation display of the NC milling process is helpful for detecting potential problems of a cutter path prior to actual machining. In the milling animation, machined shape of the workpiece is repeatedly computed and displayed after every short motion of the cutter. A single cutter motion usually removes very small portion of the workpiece, therefore most part of two successive pictures in the animation becomes the same. Based on this characteristic of the pictures, the authors develop “partial redrawing technique” for fast and smooth animation of the NC milling process. This method efficiently generates a new picture of the workpiece by properly modifying a limited part of the latest picture in the screen. The picture area to modify is determined based on the removed shape of the workpiece. An experimental system is implemented and some animations of complex milling processes are demonstrated.