If we go to the past texts with the help of grammar, we will find that it definitely plays a significant role in understanding and interpreting our texts. The "Genitive cases" may not be far from these problems. Of course, the main aim of this article is to know the genitive case and analyze it from the perspective of historical grammar based on the Masnavi in order to determine the style and register properties and to identify the secret of its charm and its distinction from other works.  The genitive case, connection between the possessed and possessor, and the manner of connection between and formation of post-modifiers are topics that have been discussed in detail. The method employed in this research is descriptive-analytical of a case study type. The target population is the Six Books of Masnavi and the data has been collected based on library studies. Although the method of this research is descriptive-analytical, and the writer provides a clear and accurate picture of noun phrases with post-modifiers in the Six Books of Masnavi and presents a meaningful report of its linguistic features, by its nature it is a qualitative research; because the researcher has tried to present novel points in this categorization and repeat many parts so as to reach a correct and logical result. In summary, in the light of this study, the use of the genitive case, its case markers, and forms are determined in Masnavi sentences; the various connections between the possessed and the possessor with Rumi's special and favorite vocabularies and combinations are clarified; finally, the manner of affixing and the patterns of noun phrases are among the cases that show the style and register of Masnavi and Rumi.