Collaborative writing is widely researched for its effects on students’ writing accuracy; however, previous research studies fail to prove its effects on students’ writing fluency. Also, none of the previous studies have investigated the framework for students to compose argumentative essays collaboratively. The purpose of this article is to fill these gaps. Sixty-two sophomore English-major students at a university in Ho Chi Minh city, Vietnam, participated in the study. One difference in the treatment between the two groups was the composing stage. Twenty-seven students from the control group composed essays individually after they worked together for idea development to make an outline, whereas 35 students from the experimental group composed essays collaboratively. Data collection was from students’ pretests, posttests of both individually and collaboratively written papers, and from the semi-structured interviews. The study reveals that collaborative writing had great effects on students’ writing fluency in both collaboratively written papers and individually written papers. Also, the study invented an efficient framework for collaborative writing activities so that writing instructors could employ to their writing classrooms if they wish to.