Teaching methods to promote cooperative learning may shape mathematics teachers' roles in the classroom, requiring a shift from direct supervision to delegating authority to small groups of students. While it is widely acknowledged that mathematics teachers' beliefs play a crucial role in shaping their instructional practices and behaviors, there is limited research on how mathematics teachers' beliefs about their professional roles influence their scaffolding behaviors in cooperative learning environments. The aim of this qualitative study is to investigate mathematics teachers' beliefs about their roles in cooperative learning and the relationships between their beliefs and instructional scaffolding practices, which are highly important in cooperative learning environments. In this study, we investigate the teaching practices of four middle school mathematics teachers in Beijing, China, through videotaping of their teaching and semi-structured in-depth interviews. The results show that the participating in-service mathematics teachers held conflicting beliefs about their roles, oscillating between direct supervision (as specialist and controller) and delegation of authority (as observer and facilitator). These beliefs could be identified in different patterns of teacher-student interaction, including unidirectional interactions, non-scaffolding interactions, and scaffolding interactions. In this paper, we critically examine these findings and conclude with a reflection on the limitations of this study and its implications for future research and practice.