After studying 50 languages, Keenan and Comrie (1977) introduced the Noun Phrase Accessibility Hierarchy Theory. This theory claims that in order to form relative clauses, languages follow a universal hierarchy. Based on this, language learning researchers introduced a hypothesis using which the learning difficulty of the relative clause in the second language can be predicted. This article attempts to determine the learning order of relative clauses in Persian as a second/foreign language through a study of the interlanguage of non-Iranian Persian learners. To this end, after examining 493 written texts from the non-Iranian Persian learners' corpus of Sa'di Foundation, sentences with relative clauses were labeled. Then, the relative clauses were sorted into 6 levels of basic, pre-intermediate, intermediate, upper-intermediate, advanced, and upper-advanced, based on the students' levels, and all samples were classified based on the relative clause heads. An analysis of the data collected from the 361 relative clauses demonstrated that the learning of the Subject and Object relative clauses exactly follows Keenan and Comrie's Accessibility Hierarchy, while other relative clauses indicate evidence of non-compliance with this hierarchy.