Due to their structural complexity, English relative clauses (RCs) are difficult to acquire for EFL learners. This study tested the predictions of seven major hypotheses proposed on the difficulty order of SS, SO, OO, and OS RCs for Persian EFL learners with different levels of English proficiency. Data was collected from 39 university students aged between 18 and 22 who performed a sentence comprehension task which consisted of 20 items involving reversible animate head nouns, with 5 items representing each of the SS, SO, OO, and OS RCs. Results showed that the determining factor in the difficulty order of the RCs for Persian EFL learners is the role of the head noun in the RC rather than the position of relativization, as some hypotheses predict. Moreover, Persian EFL learners opt for a linear parsing strategy in processing RC structures. Besides, Proficiency level did not bring about a drastic change in the difficulty order of the RCs.