In project-based learning (PBL), a powerful pedological framework in design education, groups of students developing prototypes towards a given design topic. The groups consist of members having different disciplinary backgrounds as away to stimulate the creativity of design outcomes. However, the impacts of the disciplinary diversity on outcomes has not been sufficiently investigated, especially in long-term design projects. In this research, we clarified the impact by investigating the intra-group conflicts among teams of different degrees of disciplinary diversities in 1.5-month PBL course whose participants were three high-diverse groups and four low-diverse groups. The intra-group conflicts of each group were evaluated through questionnaire and interview to group members at three phases of the course over the periods of the projects. The final design outcomes were evaluated in term of creativity. The questionnaire results show that highly diversified group has a significantly lower degree of interpersonal conflicts than the lower one in the middle phase of the course. It also suggests that higher degree of conflicts significantly leads to the higher quality of design outcomes. However, the degree of disciplinary diversities in a team did not directly have significant impacts on the creativity of design outcomes. The interview results imply that lower degree of disciplinary diversity created a minority party, which suppress the minority party to express her opinions during the team.