Different teacher characteristics affect their teaching in the classroom. Among these characteristics, teacher self-efficacy (TSE) can be mentioned. This study aimed at exploring the impact of different forms of TESOL/CELTA certification course (i.e., face to face and online) on Iranian non-native English teachers’ self- efficacy (TSE) perceptions. İn so doing, a quasi-experimental pretest and posttest design was used. The participants, who were selected through convenient and snowball sampling methods, consisted of 50 male and female Iranian EFL teachers whose majors were Translation and literature in English as well as Teaching in 30 to 40 age range, with one to more than ten years of experience in teaching. To measure the participants’ TSE perceptions, the Teacher Sense of Efficacy Scale (TSES), an extended model of the Ohio State Teacher Efficacy Scale (OSTES), was utilized (Tschannen Moran & Woolfolk Hoy, 2001). Data analysis was conducted at descriptive and inferential levels. As proved by the results of data analysis, the participants significantly improved in terms of their overall TSE perceptions as a consequence of exposure to the CELTA or TESOL certification course. Moreover, the findings revealed that although both face to face and online groups showed a remarkable improvement in their TSE perceptions after CELTA/TESOL certification course, there was a significant difference between the two groups’ TSE perceptions after the course in favor of the face to face group. The findings have some implications for in-service teacher training programs. Manuscript profile