Reconstruction of the historic stucco mortars is an important issue for its utilization in the restoration of historic stucco either for motives completion, edge pointing, crack filling or preparation of experimental samples. Analytical study by XRPD analysis and the calculation of the molecular weights of the binding materials of the intermediate phases (hemihydrate, anhydrite and portlandite) and the final phases of the historic stucco mortars (gypsum and calcite) was done to put a new methodology for the reconstructing of historic stucco mortars. The results of the study proved the similarity among the XRPD results of the historic stucco samples and the new reconstructed ones. That may justify its use as an approach for reducing the error in the reconstruction of the historic stucco mortars.