Content and Language Integrated Learning (CLIL) is a dual-focused educational approach in which language is used as tool for the learning and teaching of content. It has been widely adopted in various contexts, and many studies have been conducted to investigate the effects of such programs on students’ language learning, content learning, attitudes and motivation. The findings of previous research indicated that some CLIL programs were successful, while others failed to produce positive results. As CLIL programs is interdisciplinary, one of the factors that affect the success of CLIL is the collaboration between language teachers and content teachers. However, very few studies were conducted to document the collaboration process of CLIL teachers. Thus, the present study explored the collaboration between English teachers, the collaboration between English teachers and content teachers, and the participating teachers’ professional development in a CLIL program through observation, interviews, and analysis of documents. The participants were 2 English teachers and 7 content teachers in a primary school in Taiwan. Through analyzing qualitative data, the present study found the frequency of CLIL teachers’ interdisciplinary meetings, the participants in the meetings, and the interaction between English teachers and content teachers in the CLIL classroom had changed over time during the implementation process. Moreover, the participating teachers gradually increased their understanding of the important concepts related to CLIL and integrated these concepts into the design of lesson plans and teaching materials. Based on these findings, some pedagogical suggestions for teachers to implement CLIL programs and collaborate in such programs will be provided at the end of the paper presentation.