Vision sensor was used to track the weld line of the compressor case. The compressor case was feed in fig equipped with the rotating system, and two torches having one degree of freedom was applied in automatic welding system. The radius of rotation for the compressor case is varying with each rotating angle, while, the angle velocity is constant. Therefore, an algorithm to expect the feature of the compressor case for versing rotation angle is needed. To over come the avove difficulties, the curve fitting and composite curve were used. The experiment to verify the proposed algorithm showed desirable results for tacking the welding line of compressor case.