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