This study aims to examine 40 EFL Iraqi learner's comprehension and production of English relative clauses to know the extent to which they are aware of the structure of relative clauses in English by measuring their ability to comprehend and produce this structure. It also intends to test which relative pronoun among four pronouns we investigate is the easiest to produce and which is the most problematic. To accomplish this purpose, two types of tests were used 1. Sentence Combination Test(SCT) to measure the participants' ability to comprehend and produce correct relative clauses in English and 2. Grammatically Judgement Test(GJT) to examine whether the participants are able to recognize if the sentence with relative clauses is correct or not depending on their understanding to different types of relative clauses that were described by Kennan and Comire (1977). The results on SCT by using implicational scaling and applying the formula to find the coefficient of reproducibility were (85%) for high level and (61%) for low level. As it was shown the high level affected the participants' answers on the test. While on the GJT the results were approximately closed (61) out of 80 items for high participants were correct and (57) out of 80 items for low participants were correct so the difference between two groups are not significance. The number of errors were made by high participants less than that made by the low group. The coefficient of reproducibility for both group was calculated by using an implicational scaling and it was 96.5% which indicates that the participants in both groups are almost aware of English relative clauses
