Numerous studies have demonstrated the significance of teacher self-efficacy and highlighted its strong association with benefits not only for teachers (performance, well-being) but also for students (achievement, motivation). The study is an attempt to explore self-efficacy sources of in-service teachers. To achieve this aim, 8 in-service teachers with different positions in a school in Min Bu, Myanmar were recruited to conduct an interview. Interview guide was developed based on the literature related to sources of self-efficacy. The collected interview data was analyzed using thematic analysis. The findings denote that good teaching experience, ideal teachers, life experiences, motivation to become a teacher, positive feelings about being a teacher, principal’s lead, professional development activities, received feedback on their teaching, and work experience act as sources of their self-efficacy.