Abstract: In order to detect CO₂ arc welding seam, a new image segmentation method based on transition region is presented. Transition region is a special region located between the object and background in the real image, whose histogram has a wide and evenly hollow between two peaks.First, transition region of original image is determined by calculating the average grads of nonzero pixels of the image obtained through top-cut and bottom-cut transform, and then threshold can be obtained easily from transition region.It overcomes the effect of disturbance.Examinations indicate that it is a good method to detect CO₂ arc welding seam.