Effective writing pedagogy in higher education has been a consistent goal of researchers and instructors in the English as a second / foreign language writing practices. Formative peer feedback, a key factor in pedagogical writing practices, has been receiving growing interest (Hu & Lam, 2010). While much remains unknown regarding how the quality of peer feedback and back-feedback affects students’ writing performance, this quantitative study adopted a quasi-experimental control group design to investigate primary pedagogical effects of peer feedback on university students’ writing performance in an EFL context. A total of 198 sophomores majoring in English took part in a 15-week research study. A one-way repeated measures ANCOVA analysis was conducted to examine the comparative effect between the traditional and peer feedback groups. The result revealed that beginning with slightly different writing ability, the peer feedback group significantly made more growth in writing achievement than the traditional feedback group. Further analysis through hierarchical multiple regression analyses showed that both the quality of students’ feedback and the quality of students’ back-feedback were significant predictors of students’ writing performance. Additionally, the quality of students’ back-feedback had a slightly larger impact than that of the quality of students’ feedback. This study provides not only further evidence of the power of formative peer feedback as an effective tool to maximize learning, but also recommends the inclusion of it in the university curriculum to encourage students to direct and monitor their own learning processes and be life-long learners.