Relative clauses play a crucial role in improving students’ writing and speaking performances. In this study, a 20-question survey was delivered to 100 EFL seniors at a university in Vietnam to find out the extent to which they understood different types of relative clauses and different relative pronouns. Also, the frequency at which these relative clauses and pronouns were used in their written and spoken communication was investigated. The findings showed the participants’ lack of understanding of these aspects. They also preferred using restrictive relative clause over non-restrictive clause, and certain rel- ative pronouns were used significantly more frequently than the others. Recommendations for teachers to raise students’ understanding of relative clauses and relative pronouns, as well as to improve students’ performance were also made for teachers’ conventional teaching practice.