Öz The main purpose of the study was to investigate a) the self-efficacy level of learners of English as a foreign language (EFL) in the process of learning English, b) whether their self-efficacy levels differ according to variables, such as proficiency levels and gender. It also targets c) to explore how self-efficient EFL learners are in terms of listening, reading, writing, and speaking skills. In addition, the study also aims at d) investigating whether there is a relationship between the self-efficacy level of Turkish EFL learners and their academic achievement. The setting of the study is a preparatory program. This study was implemented with 525 students enrolled at a state university in Turkey and carried out during the spring semester of the 2016- 2017 academic year. The study followed a mixed-method design; first, quantitative data was collected through a questionnaire, “Questionnaire of English Self-efficacy”, developed by Açıkel (2011) and adapted by the researcher. The results of the questionnaire were analysed via Statistical Package for the Social Sciences (SPSS) 22.0. Secondly, semi-structured interviews, whose protocol was developed by the researcher, with randomly selected 24 participants were carried out to assist the quantitative data with the qualitative one.