Over the past two decades a growing body of international research suggests that instructional leadership from the principal is essential for the improvement of teaching and learning in schools. However, in many parts of the world, the practice of instructional leadership remains both poorly understood and outside the main job description of the principal. Thus, in many nations, the expectation for principals to act as instructional leaders represents a major change from traditional practice. The current study explores the principal's changing role as an instructional leader in Thailand, where education reforms adopted in 1999 sought to change modal approaches to teaching and learning as well as school management. The study employed surveys of principal instructional leadership conducted prior to and following adoption of Thailand's National Education Act 1999 to assess change in principal practice. The results suggest that despite new system expectations for principal to act as instructional leaders, the predominant orientation of Thai principals remains largely unchanged. The authors recommend that more systematic and substantial steps are needed to train and support principals in making this change in their role.