The present study aimed at investigating the relationship between Iranian EFL, English as a foreign language, male and female instructors' philosophy of education and their professional development to see whether there is any significant relation between the type of philosophy of education that teachers hold and their professional development. It also attempted to find out whether Iranian EFL instructors' philosophy of education subscales can be considered as some significant predictors of their professional development or not. Consequently, 300 Iranian EFL male and female instructors teaching at English language institutes in Mashhad, Tehran, and Bojnord, in Iran, took part in the study. The participants were asked to fill out two questionnaires, Professional Development Inventory along with Teachers' Philosophy of Education Inventory simultaneously, in order to elicit their ideas about the aforementioned issues. To analyze the data, Multiple Regression was employed. The results exhibited that "Perennialism" as one of the philosophy of education subscales was considered to have the only significant relation with Iranian EFL teachers' professional development along with being the only significant predictor of their professional development. Finally, pedagogical implications along with suggestions for further studies are discussed.