The aim of the present mixed-method study is investigating the role of SE instruction in English as a Foreign Language (EFL) teachers’ PD. To do so, 30 EFL teachers and 150 of their students in different high schools in Zanjan, Iran were selected as the participants of the study. The convenience (availability) sampling procedure was used to select the participants. The students were asked to fill out the the teachers’ PD survey, and the questionnaire of SE was filled out by the teachers as the pretest. Then, the teachers received in-service training for SE with a specified syllabus. At the end of the training period, which lasted 5 sessions in a 1.5-month course, the students completed the teachers’ PD survey, and the teachers filled out the SE questionnaire again as the posttest. To analyze the results of the questionnaires as the quantitative data, a paired-samples t-test was employed. In the qualitative phase, an in-depth interview was held with the 30 teachers concerning their ideas towards the effects of SE instruction on their PD, and the collected data was analyzed through content analysis technique. The findings of the study revealed that the EFL teachers’ SE significantly affected their PD based on the results of the questionnaires and in-depth interviews. تفاصيل المقالة