X-ray topographic contrast of inclusions in the crystal of potassium acid phthalate (KAP) has been studied experimentally and theoretically. The nature of inclusion formation was determined in view of their black and white contrast. An "Inclusion Decoration Method" for use in determining the nature of the strained regions in the crystal with very large areas has been proposed. The results together with such an analysis technique are presented in this paper. It has provided a practical way to identify the structural weakness in single crystals.