Gypsum mortar in mausoleum of Seyed Shams al-Din in Yazd, shows incomparable reliefs and molded ornaments. A notable instance of these is the partially damaged gypsum inscription that adorns the iwan of the mausoleum and extends over three sides of it. Apposite conservation of this valuable inscription would call for a structural and technological analysis before its structure-related damages and erosion process could be precisely and systematically examined and documented. Samples were obtained from both the surface and deeper layers of the inscription in question as well as the molded gypsum ornament and the fine coat underlying the latter. The samples were then evaluated using XRD, XRF, and SEM-EDS analyses before the mortars were compared. The results revealed that the interior and surface of this inscription vary structurally, which is partly related to the processing of the mortar and executing the inscription and is partly a function of the mortar erosion process. Erosion and structural alterations in the inner inscription are far more extensive than in the surface, leading to the reduced strength, a main reason for it being the permeation of moisture and soluble salts from the retaining wall to the deeper layers of the gypsum inscription.