Purpose: Students in Turkey have difficulty reaching fluency in English. Affective factors in teaching English are as important as cognitive factors. Perceived self-efficacy, an affective factor, is known to influence learning a second language. This study explores the effects of a university preparatory school's English curriculum on its students' self-efficacy beliefs to understand if the design, application and evaluation of the curriculum could be better suited to English language learners. Research Method: 426 students participated in the research, conducted with the cross-sectional survey design. An adapted psychometric scale was used to measure students' levels of English self-efficacy beliefs. In addition, an open-ended questionnaire was distributed to the participants to better understand their views on the sources of self-efficacy. The data were analyzed using paired samples t-test, percentages and frequency distributions. Findings: Findings revealed that preparatory class practices increased students' self-efficacy beliefs in all sub-dimensions. The fear of making mistakes, losing face, and not being able to express their opinions negatively affected students' self-efficacy beliefs. Applying the language in productive activities, showing effort to use English in daily life and taking responsibility within the context of the English curriculum, modelling after positive behavior of the teachers and friends, and encouraging expressions by the teachers positively affected students' self-efficacy beliefs. Implications for Research and Practice: To develop professional teacher training programs that apply more effective activities and provide better feedback and correction, it is imperative to establish better communication with the students and understand how to integrate technology in the language lessons.