Based on the concepts of Zone of Proximal Development (ZPD), this study compared the effects of feedback given from teacher and senior students on Thai EFL (English as a Foreign Language) students' writing. The research participants were 33 students majoring in English who were undertaking an English Essay Writing Course and 22 senior students who have already passed the course. The data were collected from a questionnaire, semi-structured interviews, a group discussion, and the students' writing drafts. The results showed that students found the senior feedback helpful to improve their writing drafts, and they felt comfortable to receive feedback from senior students; however, they had low confidence towards the comments. With regards to teacher comments, students found them helpful to raise their awareness in terms of tenses, grammar, and structure. However, the limitations of teacher feedback were pointed out in terms of confusing comments, stress, and timing. Finally, the analysis of the effects of feedback types on students' improvements showed that metalinguistic corrective feedback (CF) made by both teacher and senior students led to most of the effective corrections. Based on the findings, the implications and directions for future research will be discussed.