Gemination is a prevalent process in Shahmirzadi, a language of the Northwestern branch of Modern Iranian language family spoken in Shahmirzad (Semnan province). This paper presents examples of gemination in verbs, nouns, and adjectives in Shahmirzadi which occur morpheme-internally and externally. Data was gathered from 5 illiterate to Master’s level female and male middle aged and older Shamirzadi native speakers and analyzed within the framework of OT. We observed that gemination is the result of synchronic as well as diachronic assimilation in Shahmirzadi and that progressive and regressive assimilation patterns are themselves the result of two different constraint-rankings of manner and place of articulation and reciprocal-assimilation emerges out of these two rankings.