This research study investigated Turkish Tertiary Level EFL learners' recognition level of relative clauses. The study also examined the difficulty level of types of relative clause constructions for Turkish EFL learners and the effect of instruction on the learners' acquisition of English relative clauses. The data were gathered from two different tests (a pre-test and a post-test) given to 30 philology students studying at the Preparatory School at Hacettepe University. The data obtained from the pretest indicated that the participants needed remedial teaching; therefore, "relative clauses" were taught to students through six-hour form-focused class instruction. Two weeks later, the post-test was administered to the participants to assess the effectiveness of instruction on students' identification level of relative clauses. The data collected from the tests were analyzed quantitatively using SPSS. Findings showed that although most of the participants are good at identifying relative clauses, it is noteworthy that their recognition level of relative clauses changed significantly according to the various types of relative clause constructions. In addition, the results showed that the explicit instruction facilitated the acquisition of relative clauses to a great extent. Results of this study may provide English instructors and EFL curriculum designers with relevant information about teaching and learning of English relativization, and emphasizes the necessity of form-focused instruction.