Abstract: A new automatic border following method combining backtracking algorithm with multi-threshold-segmentation in border abstracting of gray images is present ed in this paper. It can reduce noise and improve the image quality. In view of the character of the contours having quite bigger gray values in three-dimensional ultrasonic CT data, we reconstruct the 3D surfaCes with borders in every slices stacking directly, then display those boundary surfaces. This approachv avoids the puzzles of dealing with branches in patch-formation approach. With this method, a quite clear 3D imange is obtained.