The article applies to the principles of the classification of the declination system in Greek language, namely, to the principle that appears in the modern grammatics of Ancient Greek. The research was based on the following grammatics: V. Brandenshtein "Greek linguistics" (W. Brandenstein "Griechische Sprachwissenschaft"), M. Meier-Brugger "Greek linguistics" (M. Meier-Brügger "Griechische Sprachwissenschaft"), P. Shantren "Historical Morphology of Greek language" (P. Chantraine "Morphologie historique du Grec"), A. Tsartsanos "Grammar of Ancient Greek" ("Γραµµατική της Αρχαίας Ελληνικής γλώσσης"), I. Stamatakos "Historical Grammar of Ancient Greek" ( "Ιστορική γραµµατική της Αρχαίας Ελληνικής").

On the basis of considered grammatics we can state that in the modern period there is given the traditional declination system with its three types, according to the stem endings of the nouns. The only difference that we can see is - different names of the declination types. In spite of this, there is the tendency of distinguishing of the noun flexion by the cases, according to the gender. (within the framework of the tradtitional declination system). In the details it’s given by I. Stamatoakos in his book "Historical Grammar of Ancient Greek", where he distinguishes three sub-groups of the second declination type, according to the gender of the noun.

The main principle of the classification of the declination system in the modern grammatics of Ancient Greek remains the linguistic principle.