Abstract: The problem of thinning processing engineering figures is discussed. Two new methods of quick thinning engineering figures are introduced. Figure skeletons can be obtained by means of line by line scanning through symmetrical erosion and one way erosion. These two methods are simple, with which internal storage is saved,processing is fast, and the figures will be shown on the screen. The operation shows that the methods are efficient for processing common engineering figures, such as accurate mechanical figures, logical circuit diagrams, technological process diagrams, geographical diagrams, etc ..