This study investigated the effect of group size and personality types in collaborative writing on students' writing ability. It employed a quasi-experimental design with non-randomized pre-test and post-test groups. Three classes consisting of 88 undergraduate students of Mathematics department at one of public universities in Malang, Indonesia participated in this study. The first class (30 students) was assigned to write explanation essays using process writing approach in pairs; the second class (28 students) completed the writing task using the same writing approach in groups of four students; the last class (30 students) wrote essays individually. The teaching to the students in the three classes lasted 10 meetings including pre-test and post-test. The students working in pairs and groups in particular were required to answer personality type questionnaire to determine their tendency of being extrovert or introvert students. One-way ANOVA and independent sample t-test were employed to compare the students' post test score. The results of study showed that students working in pairs and groups had better writing ability than those who worked individually. It also revealed that students working in pairs outperformed those working in groups. Finally, there were no significant difference in the writing ability between extrovert and introvert students who worked collaboratively, either in pairs or in groups.