A novel subpixel edge detection algorithm of roof edge based on symmetry center of figure is proposed by analyzing characters of roof edge. A conclusion is obtained that the symmetry center of the gray function figure is the edge of roof edge. Subsequently,the discrete form of the algorithm is deduced. Validity of the algorithm is testified by experiments in noise and no noise situations,and the reason of making error is diseussed. Experiments show that the algorithm has high de tection accuracy,less computing cost than others and the ability of restraining noise.