Teaching and learning English in international programs in an EFL nation as Thailand are uneasy tasks for both teachers and learners since English courses are provided parallel with content courses that use English as a medium of instruction. It causes an urgent demand of high level of English proficiency applying in the study; therefore, not only English communicative skills are required, but also English academic skills are demanded. As a result, a teaching approach integrating both content and language teaching and learning as Content-Based Instruction (CBI) approach was selected to develop academic reading and writing skills of Thai EFL undergraduates studying in the international programs aiming to investigate effectiveness of CBI approach in developing Thai EFL undergraduates’ academic reading and writing skills. The study was carried out at Mae Fah Luang University, Chiang Rai, Thailand. Pre-and post-tests and a semi-structured interview were used to examine their academic reading and writing skills improvement after the use of CBI in the classroom. The research findings indicate that the students got better results in post-test of academic reading and writing skills, and the students thought that this approach of teaching enabled them learn pleasantly by not worrying too much on grammar as other academic English courses they used to study. As CBI approach enabled the students improved the skills at the end of the course and generated good feeling when learning, it can imply that CBI approach is effective in teaching academic reading and writing skills at tertiary level in this particular context.