Abstract The acquisition of relative clauses (RC) has been a hot topic in the field of linguistic research, and the informants in the previous studies are mostly native speakers of English acquiring a second language or speakers of other languages learning English as the target language. In China, while there are abundant studies that focus on the characteristics of the Chinese ESL learners' acquisition of relative clauses, very limited studies investigate the English RC learning by such minorities as the Tibetan, Uygur, and Mongolian speakers. The present study aims to address the gap in the previous research by focusing on the acquisition of L3 English relative clauses by native Mongolian students in China. Using a variety of testing materials and statistical devices such as SPSS, this study attempts to evaluate three famous predictions in the field of English relative clause, namely, Kuno's (1974) Perceptual Difficulty Hypothesis (PDH), Keenan and Comrie's (1977) Noun Phrase Accessibility Hierarchy (NPAH), and Hamilton's (1994) SO Hierarchy Hypothesis (SOHH). Data were collected from sixty EFL college native Mongolian students at two different proficiency levels who participated in both the comprehension and production tasks, namely, sentence combination task, sentence separation task, and multiple-choice task. The major findings of the present study are: firstly, neither NPAH nor SOHH are completely reasonable for Mongolian EFL learners' RC accessibility hierarchies. Secondly, the results from both groups of informants mostly support the prediction of Kuno's PDH. Thirdly, the specific results vary according to the tasks that the participants completed. The study paves the way for further research on Mongolian speakers' acquisition of English syntax. Key words: English, Mongolian, relative clause, processing difficulty, accessibility hierarchy