The aim of the paper is to present a segmentation method based on an extension to multi-gray-level images of the thresholding, named multithresholding and on a merging of so obtained regions using general knowledge concerning visual data. The system implemented firstly preprocesses the digitized images. Then, after a quadtree coding, detects the thresholds by gray-level histogram processing and obtains the regions by a labeling method. Finally the system performs the merging process, in which the detected regions are merged in order to refine the image segmentation. The merging is conceived as knowledge-based process oriented to perceptual concept utilization.