Plasterwork, as an inseparable part of traditional architecture, holds a special status. One of the masterpieces of Islamic-era plaster art is the mihrab adorned with plaster decorations featuring arabesque motifs and Kufic inscriptions. The historical mihrabs of Malek Mosque (also known as Imam Khomeini Mosque) in Kerman, recognized as the oldest mosque in Kerman, are exemplary cases. The delicacy of execution and the complexity of the motifs of these mihrabs have distinguished them as unique works in the history of Islamic art. This research aims to investigate the technological characteristics of the plaster mortar used in the historical mihrabs of Malek Mosque. Studying these mihrabs is significant because understanding the composition and structure of historical mortars can greatly contribute to improved conservation, methodological restoration, and precise reconstruction of these Islamic architectural elements. In this study, three samples from three different rooftop mihrabs of the mosque were analyzed using advanced laboratory techniques, including X-ray diffraction (XRD) and scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy (SEM-EDS). The results indicated that the predominant phase in all samples is gypsum. Furthermore, one of the samples, in comparison to the others, exhibits the most homogeneous particle size distribution and the densest microstructure. Additionally, XRD analysis confirmed that this sample contains the highest amount of quartz and calcite, which is likely the reason for its greater strength and durability compared to the other samples. This sample, identified as the optimal structure, serves as a benchmark for producing stable mortars for conservation projects. These findings not only enhance the understanding of plaster manufacturing technology during the historical period but also provide an essential reference for formulating compatible mortars in the restoration of plaster artworks.