Teachers or lecturers can be one of the major influences on students’ learning in the Thai context. It is obvious that there is a major shift in teaching and learning when decided to use English as the medium of instruction. Obviously, active teaching and learning behavior is necessary. Besides possessing the academic knowledge of the field, acquiring the ability to use the first language and the target language interchangeably is a desirable quality of the English-medium instruction lecturers. It is agreed that becoming English- medium instruction lecturer is quite demanding in its nature; therefore, theses lecturers need a support system to help them achieve their expected roles. It is important for the English- medium university to recognize their needs and further assist them in a form of professional development. Several studies in European context have also supported that staff development for English-medium lecturers is extremely important and is very urgent to the situation (Vinke, 1995; Klaassen, 2001). Furthermore, Uys et al. (2007) reveal that professional development is needed for both pre-service and in-service English–medium instruction lecturers in southern Africa. As for the situation of the English- medium instruction in the Thai context, Iemjinda (2007) report that adopted professional development program for the in-service lecturers has been questioned regarding its practical outcome because lecturers fail to apply the new knowledge and skills in the classrooms. He further suggests that six key features of professional development need to be reflected upon prior the enactment of the future professional development program. The six key features involve perceived need, clarity, complexity, workability, implementation and support, and advocacy. Despite some negative report on adopting professional development in the Thai context, it is strongly suggested that the issue of the professional development of the English–medium instruction lecturers in the Thai context continue to need more reflection and research.