The study aimed at investigating the effects of peer feedback on students' English writing ability in L2 writing class. A mixed-methods research, an embedded experimental design was employed, making use of a writing pretest and post-test and self-written reflection in the experiment. Data were quantitatively analyzed through a dependent simple t-test, and content was thematically analyzed for qualitative data. The participants were 21 undergraduate students majoring in English in the three southernmost border provinces of Thailand. The findings revealed that from the mean scores of the pretest and post-test, the students had made significant progress in their writing ability. Additionally, the effect size was calculated at 1.97, which means that its magnitude was "large". Moreover, students reflected that peer feedback was a worthwhile experience for social interaction, and provided them with perceiving the writing process, developing affective strategies, supporting critical thinking skills, and developing socially and intellectually by means of working collaboratively. In addition, it helped them practice to become more autonomous learners. As a result, peer feedback should be implemented in L2 writing classes.