While the beliefs and practices of teachers on professional development (PD) are generally considered to promote quality in education and raise teachers' awareness regarding teaching strategies, there is still limited research available into different PD models' effectiveness in providing teachers with practical knowledge. This research aims to explore the role of AR in Turkish EFL instructors' beliefs, and compare action research (AR), as a transformative model, with transmissive and transitional PD models. Qualitative data collected by means of semi-structured in-depth interviews were analysed by using constant comparative data analysis procedure. The participants were nine Turkish EFL instructors from a state university, selected according to purposive criterion-based sampling. The results revealed that the PD beliefs of Turkish EFL instructors conducting AR were different from those engaging in transmissive PD models in terms of access to and reflection on knowledge. The beliefs of the former group indicated strong evidence of a lifelong learning PD process, collaboration-based PD practices with intrinsic motivation, integration of inquiry based, experiential knowledge with reflective practice experiences, and a bottom up approach to PD. In contrast, the latter group showed short-term learning, and a theoretical knowledge transmission model depending on extrinsic motivation, presenting non-reflective, passive roles, with a top down approach. In this context, AR as an on-going PD activity is seen as highly effective for fostering teachers' practical knowledge.