The purpose of this study is to explore the effects of the Content and Language Integrated Learning (CLIL) training program on Thai pre-service teachers' knowledge of CLIL approach, CLIL material design and CLIL teaching and their opinions toward the training program. A mix-method with a quasi-experimental study was employed in this research. The sample consisted of 14 pre-service Social Studies teachers from a university in Bangkok, Thailand. Data collection tools of the research included an achievement test, a questionnaire and an interview. A non-parametric statistical hypothesis test was used to assess quantitative data. Results revealed that the pre-service teachers' knowledge of CLIL approach, CLIL material design and CLIL teaching improved with a statistically significant difference. The results also revealed that the pre-service teachers had favorable opinions toward the training program when it comes to training program, learning experiences, CLIL material design, and CLIL teaching. It was concluded that the CLIL training program could have a favorable effect on pre-service teachers. As a result, it was recommended that more CLIL programs be developed and provided to pre-service teachers.