This study was designed to investigate the student teachers perceptions about and benefits and challenges of using Facebook as an online peer assessment tool for the student teachers' works. The study group included 24 student teachers in science education department of a state university located in the southeast region of Turkey. A case study approach of the qualitative method was employed in the research. Semi-structured interviews were conducted to collect the data. The interviews were audio recorded, and records of all the interviews were transcribed into full text in Turkish. Collected data were analyzed using an emergent coding approach. Codes, then, were categorized to constitute themes and subthemes. The findings indicated that the student teachers were able to give objective feedback on their peers' work and engaged more actively in class after participating in online peer assessment. Additionally, the students found it exciting and productive to use Facebook as a peer assessment tool in their learning.