The main purpose of this case study was to explore the effects of implementation of the professional development peer group. The model was based on social constructivism and the researcher’s experience as a student teaching supervisor. The process of the peer development group was operated through four circular steps: writing up of the lesson plan, classroom observation, getting feedback from the peers, as well as reflecting and sharing. The participants included 11 elementary student teachers who participated in 14 sessions of the peer development group. The result of the study revealed that the 11 student teachers gained growth on professional knowledge and skills. They placed high recognition of this model. The main accomplishments included: 1.The group members were able to work together by observing and supporting one another’s practice; 2.The group members were able to conduct group dialogues and discussions, helping to clarify the blind spots and gain new experiences; 3.They acquired self-understanding and reflective thinking; 4.They could combine teaching theory with the practice; and 5. They received guidance from experienced teachers. The mentor teacher, the administration director and the principal reported that the contributions of the development group are as follows: 1.Regarding the student teacher as the active thinker and reflective “knowledge producer”; 2.Focusing on professional dialogue about teaching and leading the teacher trainee to focusing on the reflection on the teaching process; and 3.Accelerating the teacher trainee’s preparation for entering the teaching work place.