Studies on students' use of corpora in L2 writing have demonstrated the benefits of corpora not only as a linguistic resource to improve their writing abilities but also as a cognitive tool to develop their learning skills and strategies. Most of the corpus studies, however, adopted either direct use or indirect use of corpora by students, without comparing the effectiveness between the two applications. This case study seeks to develop new lines of inquiry by comparing the effectiveness and learning strategy use in corpus-based writing revision. Four Korean EFL students used introspective and retrospective research instruments in an investigation of the effects of corpus use on error correction, error correction patterns, and learning strategy use between the two approaches. While we caution about drawing a conclusion from this small case study, the needs-based approach to corpus use in L2 writing was found to be effective for restructuring the learners' errant knowledge about language use. The approach drove students to actively adopt cognitive learning strategies by performing as "language detectives." Different effectiveness and learning strategy uses were also observed relative to the corpus use contexts as well as according to student proficiency levels. We also found pedagogical implications, which will be discussed, in relation to the two different corpus applications.