Abstract: In order to measure the radius of hat spots and Gaussian spots, knife-edge method was adopted for the theoretical analysis and experimental verification. A new iterative algorithm was proposed. The distance of knife-edge is in direct proportion to the radius of spot. The corresponding scale factors were calculated. It is convenient to obtain the radius of spots by multiplying a corresponding scale factor to the distance of knife-edge. The result shows this iterative algorithm is also valid for any spot whose distribution of intensity is already known.