Objective: The aim of this study was to identify the content factors of professional development for teachers of special elementary schools for students with intellectual disability in Tehran, Iran. Method: The present study was a mixed methods research. In the qualitative part, the population was composed of all the experts of education, of which 15 were selected through purposive sampling. The statistical population in the quantitative study comprised all elementary school teachers of students with intellectual disability in Tehran (n=347), of which 200 were selected based on Cochran’s formula through simple random sampling. To collect data, semi-structured interviews and a researcher-made questionnaire were employed. Qualitative data were analyzed through thematic analysis, and quantitative data were analyzed using the partial least squares method. Results: The results revealed that the content of teachers’ professional development  consists of 10 factors: teachers’ attitudes, specialized subject knowledge, variety of learners and their special needs, design of educational strategies, evaluation methods, learning environments and models, participation and collaboration, information and communication technology, group and individual activities, and citizenship and personality education. Conclusion: In total, content factors explained about 95% of the variance in teachers’ professional development. From among these factors, learning environments and models had the highest (0.90) and evaluation methods had the lowest (0.81) factor loading