This study aims to examine the effectiveness of genre-based in-class L2 writing instruction on Korean middle school students' writing ability and their perception change toward L2 writi ng. The research was conducted with 595 middle school students in the third grade from two co-ed schools in Gwangju Metropolitan City, Korea. Five sessions of writing instruction using a dialY genre over five weeks were provided to an experimental group, while no explicit writing instruction was provided to a control group. The experimental group was exposed to 13 writing sample texts, analyzed and identified the characteri stics of the target genre, and then constructed a diaty text in collaboration with their peers. Following the treatment, both groups were required to write a diaty ently in 40 minutes atld the students' writings were rated on content, organization, and language use. The results showed that only the experimental group, provided with explicit genre-based L2 writing instl·uction, showed statistically significant improvement in each categOlY in the post-test. In addition, a set of questionnaires and interview data revealed that the participants' attitudes and perceptions toward L2 writing were positively affected.