The purpose of this study is to reveal whether or not the social skills teaching program based on the direct instruction approach is effective on the ability of mentally retarded students in regular classroom settings to gain social skills such as apologizing, asking for help and finishing a task on time, and to generalize these abilities. This study used the model of multiple-probe design between subjects which takes place in the research methods of single-subject design. The researchers carried out this study with one 12-year-old male student and two female students, ages 12 and 11. To collect study data, this study used the teacher interview form, the social skills checklist, criterion-referenced measurement tools and data record tables. Graphical analyses were used to analyze data. To use target social skills (skills of apologizing, asking for help and finishing a task on time) in education, instructional plans that included acting as a model, guided practice and independent practice steps, which are the basic stages of the direct instruction approach, were prepared. In the social skills program, teaching sessions were held separately and conducted 3 days per week. At the end of the teaching sessions, generalization sessions took place. After this study, it was seen that the social skills teaching program based on the direct instruction approach was effective on the ability of three mentally retarded students to gain the target social skills and to generalize these abilities.