Recognizing that graduate students seldom have the opportunity to participate collaboratively, either in providing or receiving feedback to improve their academic writing skills, this study reports on the design of a computer-supported collaborative learning (CSCL) system used to investigate how graduate students transform and construct their academic knowledge through peer feedback when involved in summary writing. A sample of 24 graduate students who studied English as a foreign language was grouped into experimental and control groups, 13 and 11 graduate students in each group. The results of this study reveal that integrating the three key elements of a CSCL system facilitated improvement in the experimental group's summary writing. These key elements included (1) acquiring a basic understanding of main ideas by requesting keywords as scaffolds, (2) explicitly observing more proficient peers' writing processes when providing and receiving peer feedback (knowledge transformation), and (3) solving writing problems by revising their own summaries following peer feedback (knowledge construction). Based on the three key elements, the graduate students in the experimental group made more local (i.e., grammatical) and global revisions (i.e., text development, organization, and style) on their own as well as their peers' summaries, compared with the graduate students in the control group. The effectiveness of online peer feedback on summary writing, in transforming and constructing academic knowledge, is further discussed in this study.