As the importance of online collaborative writing is recognized as an effective teaching strategy, studies have investigated how factors such as gender, ability, and group size affect learners' collaboration. However, few studies have explored the effects of group formation, i.e., student-chosen groups (SCGs) and teacher-assigned groups (TAGs) in online co-writing. Using the framework of the Toulmin model of argumentative writing, the current study investigated whether SCGs and TAGs differ regarding collaborative approaches and characteristics in co-argumentative writing using an online writing platform (i.e., Shimo). Seventeen fourth-year English majors participated in this 2-week study, nine of whom formed into three three-person SCGs, while a teacher assigned the remaining participants into two three-person TAGs and one two-person TAG. The archived discussion notes in Shimo along with semi-structured interviews comprised the data for analysis. The findings revealed that TAGs and SCGs had different collaborative approaches. TAGs were identified as less active, while SCGs tended to collaborate better in the co-writing processes. Implications for online classroom applications and future research are discussed.