The objectives of this research were to investigate: 1) the effects of the teacher training program on the knowledge of ASEAN content and the CLIL approach; 2) the attitudes towards teaching English with ASEAN content and the CLIL approach; and 3) the application of the knowledge in English language teaching. The population comprised the secondary English teachers of the Thailand Local Administration. The two sample groups involved in the two research phases of this study included 269 participants in Phase 1 – the teacher training program for teaching English with ASEAN content and the CLIL approach and 20 volunteers in Phase 2 – the follow-up study of the application of the learned knowledge in their English language teaching. Descriptive and referential statistics were used to analyze quantitative data from the tests and questionnaires in Phase 1. Follow-up quantitative data were analyzed. Content analysis was used to systematically compress the qualitative data from focus group discussion in Phase 2. The results revealed that: 1) the teachers attending the training program manifested the knowledge of ASEAN content and the CLIL approach in pre- and post-test significantly differed at the level of 0.05, with the mean scores of 9.79 and 13.04, respectively; 2) the teachers had positive attitudes towards teaching English with ASEAN content and the CLIL approach, rating the gained knowledge and understanding at the moderate level, while rating satisfaction with the training as the top 3 ranks including the knowledge and the presentation of the trainer and the true applicability of the training content at the high level; and 3) the teachers applied the learned knowledge in their English language teaching at the high level – with 90% using ASEAN content in their teaching, 85% using it to organize classroom activities, and 40% using it to design lessons and write lesson plans – with 80% using the CLIL approach to organize classroom activities, 65% using it in their teaching, and 50% using it to design lessons and write lesson plans, and the teachers required additional training about the application of the CLIL approach.
