This paper focuses on the impact of an asynchronous online discussion forum on the development of students’ ability in and attitudes toward writing in English. To do this, 60 undergraduate students majoring in English were assigned to two experimental and control groups while receiving different types of feedback. Students in the experimental group were required to take part in an asynchronous forum by writing a five-paragraph essay, challenging their ideas and providing feedback based on a pre-designed feedback sheet, whereas students in the control group received traditional face-to-face feedback by the teacher. Analysis of the data obtained through a pre-test, a post-test, and a survey indicated that students’ writing improved significantly in the experimental group, on both lexis and grammar planes, as the results of engaging in replying to peers’ questions, involving in writing by exchanging feedback, and employing self-assessment strategies to revise their own work. Furthermore, they also expressed more positive attitudes toward writing. The study offers implications for L2 writing instruction in EFL contexts
