Ellipse detection is a widely used technology.Since there are many objects are similar to ellipse,so ellipse detection becomes a key technology in image segment and object extraction.The geometry of ellipse is analyzed in this paper.Then an ideal ellipse of a similar ellipse region can reason out in the help of the concept of inertial square.This algorithm decreased computation greatly.If the center of an ellipse can work out,an ideal ellipse of a region can be figured out.After getting the ideal ellipse,by fitting the edge of the region to the ideal ellipse,whether the region is an ellipse region can be verified.