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In recent decades, processing relative clauses (RCs) and studying their complexity in different languages had an important role in linguistic and psycholinguistic researches. This study is to compare the complexity of two types of Persian RCs: Subject-Subject (SS) type and Subject-Object (SO) type. This complexity is assessed on the psychological criterion of the preschool children’s understanding of the sentences in which these types of RCs are used. 96 Persian native children in three different age ranges of 3-4, 4-5, and 5-6 (each range 32 children) took part in this study. They were presented by the fore-mentioned types of sentences in two different forms of declarative and interrogative sentences, in different ways of showing pictures and realties, and also asking them to follow the order. Then their understanding of these sentences was tested. The results showed that SO type RCs, both in declarative and interrogative forms, were clearly more complex than SS type RCs.