Abstract: This paper studies and gives a filling algorithm of polygons against a circular window, which serves the needs of filling and clipping functions, as well as clipping only functions.It considers the edges of polygon P as directed lines,and solves the problems of deciding and filling the area of P clipped by C,based on the intersection point conecpt of in-edges and out-edges of the polygon vertices.It contains the clipping function in its filling process,and therefore can be used directly for polygons unclipped.