Abstract: The subject of this research is the typological characteristics of declension system in the Russian and Armenian languages and their diachronic changes. The author compares the modern Armenian and Grabar (classical Armenian) language, as well as Old East Slavic and modern Russian language. The goal of this article is to determine typological peculiarities of grammatical case systems of the Russian and Armenian languages in their current state, taking into account the vectors of evolutionary development of these systems in the history of the two languages. Research methodology leans on the traditional concepts of morphological typology and systemic analysis of language types proposed by G. P. Melnikov. It is established that unlike the Russian language, the Armenian declension system contains certain agglutinative and analytical features. Emphasis is placed on the detailed analysis of these phenomena and explanation of their consistent nature. The acquired results are valuable for typological description of the Russian and Armenian languages, as well as for teaching these languages to non-native speakers. The presented materials broaden scientific representations on multifacetedness of development of fusional declension on the example of two quite different languages within the same language family.