The current study investigated the quality of feedback produced via anonymous online peer feedback activity in a Thai EFL writing classroom. It also explored how the students perceived the anonymous online peer feedback activity. Peer feedback tasks, questionnaires, and an interview were used to collect the information. The results from the peer feedback tasks showed that the quality of peer feedback significantly improved. The results from the questionnaires showed that the students agreed that the online peer feedback activity helped them to improve their writing although there were some problems that should be improved. The follow-up interview revealed that the anonymity of the writers or the feedback givers would not affect how most students would react to the writings or feedbacks. It could be noted from the study that sufficient training must be provided before implementing the anonymous online peer feedback activity. The findings provide new evidence for scholars and instructors who are interested in implementing an anonymous online peer feedback in a Thai classroom.