In Thailand, the new educational policy is mandated to encourage content teachers to integrate English in content classrooms. The policy has created tensions and misconceptions among content teachers, who must change the medium of instruction from Thai to English. This paper presents an attempt to foster teacher knowledge about the Content and Language Integrated Learning (CLIL) approach in a teacher professional development program. Classroom language in English and CLIL classroom structure were provided for 15 teachers at a secondary school. Four volunteer teachers were observed to determine the extent to which teachers implemented CLIL. The findings revealed the teachers partly implemented classroom language in English and the provided CLIL structure was not evident. The contribution of this paper is to the literature of CLIL teacher professional development effectiveness and the implementation fidelity of a professional development program.