Background: Syntax has a high importance among linguistic parameters and the prevalence of syntax deficits is relatively high in children with language disorders. As such, independent examination of syntax in language development is of paramount importance. In this regard, Iranian language pathologists are faced with the lack of standardized tests. The present study aimed to determine the most frequent and essential morphosyntactic structures of the Persian-speaking children aged 4-6 years, as an initial step in the design of a test to assess their expressive morphosyntactic features.Methods: The present descriptive qualitative study was designed and conducted in two phases in Tehran (Iran) during 2014-2015. The first phase involved an extensive review of the Persian grammar sources, language development texts, modeling a test called SPELT-3, and morphosyntactic analysis of samples of spontaneous speech from 30 Persian-speaking children aged 4-6 years. In this phase, 30 structures were extracted as the most frequent morphosyntactic features used by children aged 4-6 years. The second phase of the research involved directed content analysis via in-depth and semi-structured interviews with 10 specialists in the fields of linguistics, language, and speech pathology.Results: In total, 30 morphosyntactic structures were extracted in the first phase of the study as the most frequent morphosyntactic structures of the Persian-speaking children aged 4-6 years. The overall validity of these structures was estimated at 70%. Conclusion: Based on the results, the selected morphosyntactic structures could be the foundation for morphosyntactic assessments in Persian-speaking children aged 4-6 years.