Wood knot image obtained by X-ray was done gray transformation. The contrast of the image after gray transformation can be enhanced obviously and the position of knot can be highlighted. Binary processing was adopted for the image after gray transformation. The defects areas of the binary images are filled. Then, invert the best binary image of wood defect, and then adduct the obtained image. The result of wood defect image plus is that defects regions take apart completely with their background, which means the image segmentation is completed. Wood utilization is improved. The experiment results show that this method is feasible and effective.