Studies confirm that professional development at the school base through the teachers' collaboration is the most result-oriented outcome among the alternative ways of teachers' professional development. One of the declared goals of the centralized system of teachers' professional development and performance assessment operating in Georgia is to facilitate teachers' collaboration. However, the contribution of the mentioned system in strengthening the teachers' collaboration has not been studied so far. The purpose of the present research was to study the impact of the centralized system of teachers' professional development and performance assessment on teachers' collaboration at general educational institutions. The issue was studied using the qualitative methods -- focus group, interview, and content analysis of documents. Eight schools were selected for the study. Two focus groups were conducted with teachers and one interview was recorded with a school leader at each school. Furthermore, interviews with ten experts in the field were recorded. It was demonstrated that the centralized system of professional development and assessment focused on developmental assessment helps to strengthen the teachers' professional relations. Activities closely related to the teacher's work, the correct attitude of the school leaders, teachers' involvement in making decisions, the existence of preset goals, working schedule and space are important for collaborative relations. Existence of unified mechanisms for professional development, mistrust among colleagues, changes being constantly introduced to the education system, shortage of time were revealed as hindering factors for teacher collaboration.