The geometric decorations are among the most widely used decorations in Islamic art, which have been used alone and sometimes in combination with other motifs or inscriptions. The plaster Mihrabs related to 6th to the mid-8th century AH, coincided with the Seljuk and IL-Khanid eras, are among the works that in addition to various plant decorations and inscriptions, are decorated with various geometric and beautiful ornaments. The goal of this research is to introduce and compare different kinds of geometric ornaments used in the plaster Mihrabs of the mentioned centuries in terms of diversity, constancy or repetition and place of application in different parts of the Mihrabs. Therefore, the main question of this research is arisen as follows: What are the geometric ornaments of the plaster Mihrabs made between the 6th to the mid-8th centuries AH and what are their similarities and differences in terms of the type of designs and their place of application in different parts of the Mihrabs made during the Seljuk and ILKhanid eras? The data was collected through field research, i. e. photography, and linear designing of the images, referring to the written resources and was analyzed through a comparative-historical method. The results, obtained from studying 40 plaster Mihrabs made during the mentioned periods shows that in both eras, the majority of ornaments are composed of numerical knots on the basis of six, like “ six and roundel” , “ six and three rounding fairies” , etc. that are mostly seen on the half-columns and opposite or side walls of the plaster Mihrabs. The geometric, zigzag, chain and wicker-like designs have been repeated frequently along the narrow margins. The most significant differences of the Mihrabs made during IL-Khanid dynasty in terms of geometric ornaments are using more knots, simple geometric figures, more detailed and diverse geometric designs along narrow margins and combinations of knots with motifs (flower and leaf).