In order for inclusive education teachers to create a high quality environment, to recognize which individuals require special education, and to implement the principles of inclusive education, these teachers need to be able to use methods and techniques related to the program’s content. In addition, student-performance evaluations need to happen and IEPs must be prepared, implemented, and evaluated by IEP teams. After examining the literature, however, teachers are observed to have insufficient knowledge about how toon preparing IEPs. This study aims to reveal the effect that the IEP training program has on how inclusive education teachers perceive their IEP competencies for the development process. In line with this aim, answers to the two following questions have been sought: (a) What are inclusive classroom teachers’ selfperceptions of their IEP competencies, and (b) Do inclusive classroom teachers’ post-test scores from the IEP Competence Scale show a significant difference based on whether or not they had attended the In-Service IEP Development Training Program? This study, being a mixed-methods design, makes use of both qualitative and quantitative methods. During the qualitative phase of the study, interviews have been conducted with 24 classroom teachers, whereas during the quantitative phase, an experimental study has been conducted over one control and one experimental group, each containing 19 teachers. As a result of the semi-structured interviews conducted in the present study, teachers are found to have shortcomings in almost every stage of the IEP development process, whereas a significant difference in favor of the experimental group was found upon examining both groups’ post-test scores following the in-service IEP training program.