The issues and the underlying motives behind second/foreign language teachers' professional development have been increasingly investigated by the experts and practitioners in the field due to its vital role in determining teachers' success. In line with such concerns, the current study was conducted to explore the underlying factors constituting the newly developed teachers' professional development scale. It also intended to find the professional development factors that Iranian EFL teachers use to enhance teaching effectiveness. 142 Iranian male and female practicing teachers were conveniently selected to cooperate in this study. They were asked to answer the 18-item Likert scale questionnaire and express their attitude about the contributing factors. Three sources of evidence were used to support the validity of the questionnaire including content validity, reliability coefficient, and factor analysis. The result of exploratory factor analysis depicted that this questionnaire consists of two underlying components: self-regulation and other-regulation components. It was also realized that Iranian EFL teachers tend to rely more on their own experiences and capacities rather than external guide and support in their professional development. Further research can focus on qualitative data collection procedures such as interviewing teachers in order to realize their views toward the factors that help them develop professionally.