This study aimed to investigate EFL undergraduate students' self-efficacy and explain English writing self-efficacy as the dominant aspect in writing the thesis. The method utilized in this study was the descriptive qualitative method. This study employed a questionnaire adopted from The Questionnaire of English Writing Self Efficacy (QEWSE) and conducted interviews using the reflective practice based on Gibb's reflective cycle theory as instruments in collecting the data. This qualitative research was conducted on eighth-semester students of the English Education Department in the academic year 2019 at Walisongo State Islamic University, Semarang. A total of 30 students participated in filling out the questionnaire, and the results were then processed, identifying nine students in the high and low categories who were subsequently included in the interview stage. This study's data analysis involved data reduction, data display, and drawing conclusions. The research findings of the questionnaire data analysis indicate that EFL students' self-efficacy in writing a thesis is high level. Among the three indicators of English writing self-efficacy, students' beliefs about their abilities to generate ideas have the highest score (dominant indicator) in constructing high writing self-efficacy for students, with a score of 64% Who choose to agree. Meanwhile, the result of interview data showed that EFL students' reflection on self-efficacy in writing a thesis has a positive reflection. Some students have a high level of self-efficacy in writing their thesis. Even though, some students face challenges when writing their thesis, including difficulties in finding relevant references. However, they can overcome these obstacles by strongly believing in completing their thesis.