The current study was to see if collaborative writing activities would impact each student's writing quality after getting engaged in an academic writing course for argumentative essays. The study enrolled 62 third-year English majors at the Faculty of Foreign Languages, Van Lang University, Ho Chi Minh City, Vietnam, 35 in the experimental group and 27 in the control group. The ages of the students ranged from 19 to 21. All teaching/learning activities, such as the selection of topics, brainstorming, or peer/lecturer feedback, applied to train the students of the two groups were similar except for the essay-composing stage. While the control-group students composed an essay individually, the experimental-group students conducted it collaboratively. That is, the group members composed an essay together. Data from pre-and post-tests of students' writing were analyzed to compare the students' writing quality. The study discovered that jointly authored papers performed much better than those written alone and that collaborative writing activity significantly affected each student's writing quality.