Usually, in different parts of Iran, according to the climate and according to local materials, suitable mortar has been selected for the construction of buildings and for rendering and architectural decorations. Studying the mortar structure of five buildings located in four different climates of Iran and related to three historical periods can complement the previous few studies regarding the technical understanding of mortar in the Sassanid, Seljuk and Mongol Ilkhanate periods. This knowledge can be valuable in three ways: knowing the technical knowledge of making mortar in historical periods; Helping to preserve the remaining these works in the studied buildings; Assisting in reverse engineering and making suitable mortar for the restoration of these works. The current research has been carried out with the aim of obtaining the parameters that have the greatest effect on the strength and durability of gypsum mortar in historical periods. In this regard, sampling was done from the studied buildings, which was evaluated and compared between these mortars by using analyses (XRD, XRF, EDS) and studying with a digital microscope as well as SEM. The results of the research show that the climatic difference and the difference of historical periods do not have a great effect on the physical and chemical characteristics of gypsum mortar. Two influential parameters (the influence of the second parameter is more) which was proved in the result of the present research, is a: The amount and how to combine materials to make mortar b: how to process the mortar. The traditional craftsmen have tried their best to reach the required quality in the mortar application stage. One of the important points that is considered for how to process and even mix gypsum mortar is the functionality of the mortar. Usually, gypsum mortar is implemented with three functions (mortar between stone and brick materials, mortar for coating surfaces, mortar for Stucco decorations), each of which can be divided into subsections, and all of these have an effect on how the mortar setting.