Motivation is an important area of knowledge and ability for teachers of English in Thai universities. This thesis addresses the concern that in many English classrooms in Thailand, students’ motivation is not optimal, despite the importance of the subject and the education policy requiring teachers to support student autonomy. A mixed methods study was undertaken drawing on the theoretical framework of self-determination theory. Three phases of research explored the dynamic relationship between teachers’ beliefs about motivation and autonomy, the strategies they used in their classrooms, and their students’ perceptions. First, a questionnaire was used to gather data about teachers’ personal backgrounds as well as their beliefs about autonomy and teaching practices in the Thai university context. Then, interviews were used to gather more in-depth data from nine autonomy-supportive teachers in different locations. Interview participants were then targeted for classroom observations to gain a deeper understanding of the teachers’ practices in relation to their beliefs about autonomy and the perceptions of their students as provided through an open-ended questionnaire. The findings revealed that teachers' understandings about the nature of autonomy were usually aligned with the idea of independence, and they believed that independent learning was the goal of a student-centred approach to teaching. Not only their beliefs about autonomy but also their beliefs about student motivation, their teaching goals, their prior learning experiences, and their autonomy influenced their actual teaching practices. Furthermore, the study found that the curriculum itself was the main constraint in the implementation of teaching practices they believed were effective. Viewed in the context of motivation literature in cross-cultural educational settings, the findings suggest that students could potentially be more motivated and engaged in the classrooms. The findings of this study offer teachers and university administrators new insights into learner autonomy and motivation as well as the opportunity to more effectively influence English language education in Thailand.