This study investigates the extent to which 50 Arabic-speaking EFL learners comprehend English relative clauses (RCs). It also aims to test which relative pronoun among the seven pronouns we are investigating is the easiest to comprehend and which is the most problematic. Furthermore, it aims to measure whether the English proficiency level of the participants affects their performance on the test. Therefore, a multiple-choice test was administered in order to examine their comprehension of this complex syntactic structure. The participants were asked to choose the correct answer out of four choices. The results demonstrate that Arabic-speaking EFL learners may not be fully aware of English relative clauses (total percentage= 48.5%). The t-test shows that the English proficiency level affected the participants' performance on the test. Consequently, there is a significant difference between the answers of the advanced learners (ALs) (61.3%) and those of the intermediate learners (ILs) (35.6%). Moreover, the results reveal that the easiest relative pronoun to comprehend is the pronoun when, and the most difficult one is the pronoun whom. This study accounts for these difficulties and concludes with pedagogical implications and recommendations for further research.