This study examined upper-secondary school biology teachers in Myanmar viewed the impact of a customized CLIL teacher training model on their English language knowledge, content knowledge, and teaching skills. Before the training, a needs analysis was conducted to shape the training topics, which were then integrated into this study's training framework based on Coyle et al.'s (2010) CLIL model. Data collection occurred during and after the training. Questionnaires, interviews, and micro-teaching self-evaluations were used to reflect the perceived impact of the model on teachers' competency development. Findings showed a positive impact of the model on teachers' subject content teaching knowledge, language knowledge and skills, as well as pedagogical skills though its effectiveness varied based on teaching experience. This study suggests that the teacher training model should address the unique needs of Myanmar teachers.