Abstract: Regarding that the traditional algorithm can only achieve a low successful defect segmentation result for the redundant background information in X-ray image,an efficient thresholding approach to confine transition region of the object based on supervision is introduced. The methods first implement gray level range-estimation,using sample images in the light of priori knowledge. According to the theory of fuzzy subset,a new descriptor taking both frequency and degree of gray level changes into account is developed. The final segmentation threshold can be determined by mean of the histogram of the transition region. The proposed method was compared with their counterparts on a variety of images,and the experimental results show its effectiveness.