This qualitative study explores the impact of Differentiated Instruction (DI) professional development course on the cognition of Thai English as a Foreign Language (EFL) teachers. Focused on two in-service secondary school teachers, it examines how DI training influences their teaching beliefs and practices. Data were collected through interviews, reflective journals, and classroom observations. Thematic analysis shows that DI professional development shifts teachers’ methods from traditional to student-centered approaches, improving how they respond to the different needs of students. This shift changes how they view their role as teachers, from being a source of knowledge to a facilitator of learning, moving away from simply giving out information to scaffolding the whole learning process. This leads to better interactions between teachers and students. The keys to this transformation are peer feedback, reflective practice, and the practical implementation of DI in the real classroom context. The findings highlight the potential of DI training to improve English language teaching in Thailand, suggesting its inclusion in teacher education programs.
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