Supporting students with special education needs (SEN) in classroom is a main duty for teaching assistant (TA). According to the Code of Practice of Education, students with SEN in Hong Kong were encouraged to enroll in mainstream school. Therefore, TAs are inevitable to work with students with SEN. This research was an investigation on TAs' self-efficacy which was a factor that would affect students' achievements and classroom management. There were two purposes in this research. First, level of TAs' self-efficacy would be explored. Second, the effect of training course to accommodate students with SEN on TA's selfefficacy would be studied and further suggestions on training course reformation would be made. Quantitative method was adopted in this study. 463 TAs had participated in this study, and their self-efficacy had been tested in 5 domains: teaching support, learning support, behavior management, cooperation and administrative support. Data indicated that TAs were most confident in cooperation and least confident in teaching support and behaviour management. Training to support students with special education needs (SEN) were found to contribute to TAs' self-efficacy in all 5 domains. However, no differences were found among different training hours because the training course cannot meet the needs of TA. This paper suggested that future TA training course should put more emphasis on teaching support and behavior management.