Curriculum implementation has been studied from three perspectives: curriculum fidelity, mutual adaptation, and curriculum enactment (Snyder, Bolin, & Zumwalt, 1992). The research from fidelity perspective has focused on: (a) determining the degree to which a particular innovation is implemented as planned and (b) identifying the factors that facilitate or hinder planned implementation (Snyder et al, 1992). In this case study, we investigated implementation challenges from the curriculum fidelity perspective. The participants included a physical education teacher and his 3-5 grade students in a large urban elementary school involved in a curricular innovation project. The teacher was learning to teach a health-science based physical education curriculum designed using the constructivist learning principles. Data were collected through observation field notes from sixty lessons, three structured interviews with the teacher and eighteen with his students. The data we focused on were the teacher's implementation procedures, students' reaction or responses to the implementation during learning, and the teacher's reasoning during implementation. Interviews were tape-recorded and transcribed. Lesson plans were collected and informal conversations with the teacher and students were summarized after each observation day. Data were analyzed using open, axial, and selective coding procedure and data from different sources were triangulated to establish trustworthiness. One salient theme emerged. That is, the teacher's belief about physical education is a great challenge to implementing the constructivist curriculum. Although the teacher learned a lot about constructivist approach from staff development, he believed that much of the cognitive tasks students were expected to do were contradictory to the physical aspect of learning in physical education. For example, student journaling became the greatest challenge in that it is difficult for him to see the relevance of students being active both cognitively and physically. On the contrary, most students reported that they had “learned a lot” of health-related science concepts in addition to being physically active in class. The finding implies that it is not what the teacher knows challenged the implementation of the constructivist curriculum. It is his belief that physical education should provide pupils an environment where they want to “come and run and have fun” that challenged the implementation of the constructivist curriculum.Keyword(s): curriculum, physical education PK-12