The aim of this study is to analyse the effects of principals' instructional leadership on the teachers' perceptions of their self-efficacy. Mixed method, in which quantitative and qualitative techniques are employed together, was used in order to analyse the data obtained. The sample for quantitative analysis included 435 teachers working in schools in Sahinbey Province and study group for qualitative analysis included 24 teachers working in these schools. For the quantitative data, regression and correlation analysis were done; for the quantitative data, descriptive and content analysis were done. Data were obtained using Instructional Leadership Behaviours Scale (Hallinger, 2011) and Teachers' Self-Efficacy Perception Scale (Tschannen-Moran ve Woolfolk Hoy, 2001). According to the result of the study, there is a significant medium level relationship between principals' instructional behaviours and teachers' self-efficacy. In addition, instructional leadership behaviours displayed by principals contribute positively to teachers' motivation and task focality, the development of learners' ability to learn, and the self-evaluation skills of the teachers about themselves and students.