Subject-specific training in mathematics teaching for students with intellectual disabilities (IDs) has been little studied in the academic field and is uncommon in the practice of teacher education. Given that teachers face significant challenges in regard to teaching mathematics to ID students, it is necessary to know whether there is a need for subject-specific training. If yes, what kind of subject-specific training do teachers need? What kind of subject-specific knowledge should be covered? How can this kind of training contribute to improving mathematics teaching for ID students?To answer the above questions, this study focuses on one particular teacher professional development programme (BE MATHS programme) for mathematics teachers who teach ID students in special schools in Hong Kong. Programme effects and the contributing factors of the effects are examined to explore whether and how subjectspecific support can contribute to the profession of teaching mathematics to ID students.The study is conducted via a mixed-methods approach. First, it uses quantitative measures to examine teachers’ changes in mathematics teaching efficacy and students’ changes in their academic engaged time in mathematics classes. Second, a qualitative approach is used to explore the contributing factors of the changes.Quantitative results show that the programme has a significant positive effect on teachers’ mathematics teaching outcome expectancy and a conditional positive effect on teachers’ personal mathematics teaching efficacy. It is also found that the programme can improve teachers’ mathematics teaching by engaging more students in mathematics learning.The qualitative findings in this study show that the design of teaching and learning trajectories with subject experts is a substantial learning activity for teachers. During the design process, teachers and subject experts identified knowledge gaps within the mathematical content to be delivered to ID students and successfully inserted