The purpose of this study is to identify the probable factors that can contribute to prospective teachers' levels of professional self-efficacy beliefs. For this purpose, quantitative methods are used to measure their perceived autonomy support during undergraduate education, their attitudes towards the teaching profession, and their levels of professional self-efficacy beliefs, while qualitative methods were used to determine the factors affecting their professional self-efficacy beliefs. This study is carried out as a survey model, collecting data using mixed methods. The study group consists of 173 prospective teachers who graduated from a state university' s faculty of education at the end of the 2014-2015 academic year. The study' s quantitative data have been collected using the Learning Climate Scale, the Attitudes towards the Teaching Profession Questionnaire, and the Teacher Self-Efficacy Scale. The qualitative data are collected using an open-ended questionnaire. Correlation and regression analyses are used for analyzing the quantitative data, and inductive qualitative data analysis is used for analyzing the qualitative data. The quantitative data analysis shows professional self-efficacy to have a significant relationship with teachers' attitudes towards the teaching profession and their perceived autonomy support (p < .05); also, attitudes towards the teaching profession and perceived autonomy support (r 2 = 0.0209) are significant predictors of professional self-efficacy (p < .05). As a result of the qualitative data analysis, prospective teachers' selfefficacy beliefs have been determined to be affected by undergraduate education, internship experience, fondness of the profession, lecturers' attitudes, self-confidence, and societal status of the profession. The quantitative and qualitative findings are seen to be consistent and mutually supportive.