While research on peer feedback in L2 writing has proliferated in the past two decades, little research has investigated the role of language use in peer written feedback in EFL writing contexts. The existing peer feedback studies have failed to explain how the use of L1 and L2 in peer written feedback can influence the peer feedback practices. Using the peer written feedback on a survey report by Chinese EFL learners as the data, the current study aims to investigate how the use of L1 in the process of giving feedback influences the peer written feedback practices. The findings indicate that L1 could play a mediating role in the written peer feedback process and L1 was found to help mediate the cognitive resources in working memory and bring positive changes to the feedback practices. With the scaffolding role of L1, L2 learners could produce more comments on content and offer more specific feedback. Implications and suggestions regarding the use of L1 in peer feedback activity have been proposed finally.