This corpus-based study aims to investigate the grammatical properties of English relative clause extraposition (RCE) construction. To this end, we looked into the data from the corpora BYU-BNC and COCA, and extracted a total of 597 RCE instances. The indefiniteness of subject NP as well as the passive voice of the predicate is the prominent grammatical feature in this construction. The grammatical weight of the extraposed relative clause also seems to be a possible factor to trigger the RCE construction. In other words, subject NP-to-extraposed RC length ratio provides a plausible answer to the question why the RCE construction is preferred over the canonical construction. In addition, the relative pronoun which in both finite and infinite RCE constructions stays on the top in frequency rate, and any relative pronouns are prohibited from being deleted in the postposed relatives. One thing to notice is that the behavior of the infinitival RCE construction is grammatically identical with that of the finite RCE construction.