The interactive functions of weblogs facilitate computer-mediated peer reviews for collaborative writing. As limited research has been conducted on examining the training effects of peer reviews on students' peer comments, their revision quality, and their perceptions when composing in weblogs, the present case study aims to fill the gap. Thirteen freshman students participated in an EFL writing class, in which they wrote four formal assignments in weblogs. Peer review training was conducted in the second and third assignments to facilitate the collaborative process. Comparisons between reviews without and with training (i.e., the first and the fourth assignments) indicate that the students made more revision-oriented peer comments and had more success in revising their compositions, although they adopted less than 50% of the comments for revision. The students' perception data show that blog-enhanced instruction stimulated their interest in improving their writing. Yet, not all of the participants felt confident about providing useful peer feedback. With the empirical evidence presented in the study, blogs could serve as a suitable platform for EFL writing instruction concerning giving opportunities for interaction. As training is essential to make computer-mediated peer review effective, the study supports the crucial role played by language teachers when incorporating Internet technologies into writing instruction.