This paper proposes a two-step circle detection algorithm using pairs of chords. It is shown how a pair of two intersecting chords locates the center of the circle. Based on this idea, in the first step, a 2D Hough transform (HT) method is employed to find the centers of the circles in the image. In the second step, a 1D radius histogram is used to compute the radii. The experimental results demonstrate that the proposed method can detect the circles effectively. (C) 2001 Elsevier Science B.V. All rights reserved.