This study, which investigates the effectiveness of the direct instruction method implementations in teaching fractions to students with mental deficiencies, utilized a multiple-case design, one of the single-subject research methods. The study involved three students who were diagnosed with a mental deficiency, who had the prerequisite skills, and who did not have basic fraction skills. The study was conducted in individual rooms allocated to the students in the institutions they attended. Data were collected through the data collection tool including 16 behaviors concerning the basic fraction skills. The findings obtained from the study showed that the direct instruction method was effective in teaching basic fraction skills to students with mental deficiencies. In line with this result, teachers who teach students with mental deficiencies could be recommended to use the direct instruction method implementations in teaching fractions.