Many educators question the value of explicit grammar instruction in foreign language education and look for alternative ways to enhance the communicative ability. Focusing on Korean leaners’ acquisition of English relative clauses, we examine whether grammar knowledge can result from experiencing English texts without explicit knowledge being provided at the outset of the learning. Our study shows that many grammatical features of English relative clauses can be acquired incidentally without explicit grammar instruction. Further, we found that the knowledge acquired through the implicit incidental learning can be better retained than that through the explicit learning.