Content and Language Integrated Learning (CLIL) has been practiced in Harriet's English Wonderland (pseudonym) in Taiwan. This study explored four NESTs (native English-speaking teachers) and one NNEST (non-native English speaking teacher) in terms of collaboration and identity negotiation on lesson planning and material development for CLIL lessons. The data in this case study included lesson plans, interviews, and observation field notes. This study developed two major findings. First, the CLIL, lessons featured language skills grammar, vocabulary, and functions integrated into subject content areas (e.g., science and social science) with the outputs focused on hands-on activities, word and grammatical exercises, and recaps. Secondly, the NEST and the NNEST identities were shaped through the negotiation of topics, sources, and activity designs for both the input and output of CLIL lessons.