While previous studies have contributed to our understanding of the nature of peer feedback, for ESL/EFL learners the mediating effects of language in peer feedback process are under-explored. Using the peer written feedback on an essay by 22 Chinese EFL learners and interviews with them as the data sources, the current study aims to investigate the use of L1 and L2 in peer written comments and the factors that may influence students' switch between L1 and L2. The results show that EFL learners did use L1 to give peer feedback, which focused more on content and organization than that in L2. The study reveals a number of interacting factors that affect L2 learners' utilization of L1 and L2 as major mediating artifacts during peer feedback, such as L2 proficiency, student beliefs, learning goals, teacher requirements, teacher feedback practices, and power relationship between reviewers and writers. Implications for L2 writing instruction and future research are provided in the paper.