Despite a large body of literature on peer feedback and writing motivation, relatively few rigorously designed, experimental studies have been conducted to explore whether or not peer feedback affects students' writing motivation and which dimensions of writing motivation may consequently change. Using an experimental design, this study explored the impact of peer feedback on students' writing motivation among 76 university students for whom English was their second language (L2). In this study, the 76 university students were randomly divided into two groups. For one semester, the experimental group of 38 students participated in peer feedback sessions, whereas the control group of 38 students received traditional teacher assessments. At the beginning and end of the semester, both groups were given a questionnaire on L2 writing motivation as a pretest and a posttest; after the semester, individual interviews with the writing teacher and five university students from the experimental group were conducted. The results of a series of ANCOVAs demonstrated that peer feedback had significant effects on students' attitudes toward L2 writing courses and significantly improved their self-confidence in L2 writing, whereas peer feedback had little effect on students' anxiety about L2 writing. The results of the interviews corroborated the quantitative results. Such evidence can inform writing pedagogy, especially in light of a recent call for the integration of formative assessment into L2 writing instruction.