While research on peer interaction in language learning has proliferated over the past two decades, little is known regarding why students interact differently with their peers and how collaborative patterns can be developed to promote the effectiveness of small group work in L2 learning. To fill such a void in the research literature, the present case study investigates the factors that shape the collaborative pattern of a small group of Chinese EFL learners in peer feedback activities. Multiple sources of data were collected, including video recordings of peer feedback sessions, semi-structured interviews, stimulated recalls, and student drafts of writing. The findings show that students’ beliefs and values, students’ motives and goals, the use of mediating artifacts, and the power relationship among the students shape the patterns of group interaction considerably. This study deepens the understanding of the nature of peer interaction in peer feedback for L2 writing and contributes new knowledge to the research on pair and small group work in L2 learning.