The present study addresses the effectiveness of grammar classes activating implicit L1 (first language) knowledge to improve explicit and implicit L2 (second language) knowledge. Current SLA(second language acquisition) researchers agree that there are two types of knowledge in foreign language ability: explicit and implicit knowledge. Their main concern is the possibility of conversion and method of transforming explicit knowledge into implicit knowledge, which is the ultimate goal to secure for L2 learners. For the purpose of the study, 66 participants of Korean middle school students were allotted into a control group and an experimental group to see (a) how the experimental classes affect their explicit and implicit L2 knowledge. And each group was also divided into a control group and an experimental group in order to investigate (b) how much practice treatment has an influence on their implicit L2 knowledge. The data analyses revealed that L2 learners` implicit L1 knowledge was beneficial to obtaining explicit L2 knowledge. In addition, ‘practice’ showed the possibility of being instrumental in forming implicit L2 knowledge notwithstanding the failure of producing a statistical significance. (Kongju National University & Chungnam Institute of Foreign Language Education)