A new segmentation algorithm which was divided into two steps was proposed for an extended target in complex backgrounds by utilizing the K-means clustering and fractal theory.Firstly,the K-means clustering algorithm was improved by using the rough set theory to determine initial cluster centroids.On the basis of K-means clustering segmentation and region connection,the edges of the target and backgrounds were extracted accurately and intactly.After boundary tracking,the potential target regions were detected according to the characteristics of the extended target.Secondly,by giving the function of a fractal dimension changing with the scale,the natural backgrounds in potential target regions were removed by the fractal scale invariance.Then,the background conglutination was eliminated by a mathematical morphology method.The experimental results indicate that the algorithm can segment the extended target in complex backgrounds correctly and reliably,and the segmented target reserves a good contour.