In an attempt to explore effective instruction in the English as a Foreign Language (EFL) setting, this study investigated language errors identified by students and teachers in three different revision stages: self-revision, peer revision, and teacher revision. It gave the focus to the effects of the three different methods on learners' writing quality, revision behavior, and perception of revisions. The subjects were 10 students in Bangkok University who shared the similar English learning experiences. The data were collected from a writing task, a peer response sheet, and a semi-structured interview. Error numbers and error types identified in the revision stages were compared to show the differences among the three methods. This study was an attempt to analyze learners' revision behavior, and the results showed that they focused more on surface errors instead of semantic or textual ones. The study also showed that teacher revision was indispensable in learners' language output modification. It did a great help to the learners' acquisition of linguistic forms which were hard-recognized errors by themselves. Also, teacher comments were perceived as significantly more effective than peer comments. However, the students showed that peer revision had roles to play that couldn't be filled by teacher revision. The post-task interview results revealed learners' positive perceptions toward peer revision, indicating its usefulness in the EFL context. (Contains 2 tables and 2 figures.)