This paper presents a case study examining 3rd, 4th and 5th graders' design fixation and cooperative learning in an engineering design project. A mixed methods instrument, the Cooperative Learning Observation Protocol (CLOP), was adapted to record frequency and class observation on cooperative learning engagement through detailed field notes. Students' design journals and reflections were also analyzed for an inductive qualitative analysis. The findings indicate three major themes of design fixation: 1) fixation on common features of things; 2) fixation on popular teenage culture; 3) fixation on the first design idea. In the cooperative learning process of elementary engineering design project, although pupils had demonstrated some abilities to solve concrete problems in a logical fashion, the participants encountered a number of obstacles in the group. Dominance, social loafing, and other problems occurring in the group process might have offset certain benefits of cooperative learning. Implications of the findings are also discussed.