The aim of this study is to search instructors' self-efficacy perceptions in English teaching at university prep school context. The relationship between instructors' self-efficacy perceptions and their use of teaching strategies, proficiency level and background characteristics is also investigated based on the three subdimensions of self-efficacy. Mixed method design was employed in the study. In the first phase, a four parts questionnaire was conducted to 374 prep school EFL instructors at 8 different universities in Turkey. In the second phase, interview sessions were held with 25 instructors working at same universities to gain a wide perspective on the points investigated in the questionnaire. Results have revealed that participants' self-efficacy level is quite high especially in classroom management and instructional strategies. Moreover, a significant relationship was found between instructors' self-efficacy perceptions and their language proficiency. Another important finding of the study was that there was a strong correlation between instructors' self-efficacy perceptions and their teaching experience.