Teacher professional development can be referred as a way for a teacher to develop an individual’s skills, knowledge, attitude, performance, expertise and competence. One of the ways to achieve it by doing peer observation. Peer observation is an activity to preserve and to advance teachers’ quality. This research aims to explore the advantages of the implementation of peer observation according to the teachers’ perception and investigate the disadvantages of the implementation of peer observation according to the teachers’ perception. This research used qualitative research approach and employed descriptive qualitative design. This research was conducted at an English department in a private university in Yogyakarta. There were involved three English teachers as the participants. The researcher interviewed participants to get the data. The researcher also used interview guideline as the instrument of the research. The findings of this study showed that there were some advantages and disadvantages of peer observation according to the teachers’ perception. The advantages were that peer observation improved the quality of teachers, it made the teachers more aware, it made the teachers more prepared in teaching, it increased the teachers’ knowledge about teaching, it increased their teaching techniques and it made teacher feel happy and enjoy. The disadvantages were that peer observation made teacher feel bored, it made teacher feel anxious, it made teacher feel awkward, it made teacher feel intimidated.