This study aimed at investigating the impact of problem-based learning (PBL) on students' writing skills and the process of teaching and learning. An embedded mixed-method design was applied involving 41 seventh grade students of SMP Negeri 4 Denpasar. In this study, the quantitative data were students' writing skills which were collected through writing test. The students' writing were scored by using analytical scoring rubric. Then, the acquired data were analyzed statistically by using descriptive statistics and paired sample t-test. Meanwhile, the qualitative data were obtained from classroom observation which was facilitated by field notes; and questionnaire administration which was facilitated by open-ended questionnaire. The results revealed significant difference in students' writing skills before and after the study. Problems were found to stimulate them to produce better content and organization of writing. During PBL activities, students were observed to actively share ideas and work collaboratively in composing their writing. Such a classroom situation has brought an implication that PBL obviously managed to improve students' collaborative learning skills.