The topic of this study is to analyze errors made by non-English majored tertiary students by using relative clauses. The study aims to find out difficulties encountered by the non-English majored students in using relative clauses. To achieve the purposes, the researchers applied the design of descriptive research and used convenient sampling to choose the sample of the research. 42 non-English majored university students who were currently taking courses at MEnglish center were selected as participants of the study. To answer the first research question, the tests were distributed to the students online via the support of Google form to detect their errors. Then, the researchers continued to employ the semi-structured interview to figure out the answer to the second research question. Eleven participants were willing to take part in the interview which focused on their difficulties in using relative clauses. The results of this study showed that the majority of non-English majored tertiary students failed to recognize the use of the relative clause "where". In terms of their difficulties, these include the failure to grasp the use of "that", the lack of vocabulary competence, and the influence of translation from English into Vietnamese. Keywords: relative clauses, common errors, difficulties, non-English majored students