Following the Bilingual 2030 policy in Taiwan, the Ministry of Education began to promote the implementation of bilingual teaching in some subject domains of elementary and secondary schools in 2018; the purposes were to develop students' ability to use English for learning domain-knowledge as well as cultivate their practice of bilingual learning and thinking. This research focused on bilingual teaching in "integrated activity curriculum" of elementary schools for literature review, and further included a teacher at one suburban elementary school in the east region of Taiwan as a study case to introduce and discuss his teaching practice. Based on the analysis of interviews with this teacher, it was found: 1. The use of bilingual teaching in "integrated activity curriculum" of elementary schools can provide students with learning integration, exploration, and innovation of language and domain-knowledge; 2. Bilingual teaching in "integrated activity curriculum" of elementary schools needs to integrate the resources of both in- and out-of-classroom for the planning and implementation of formal and informal curriculum.