This paper aims to reveal a teacher's actions to participate in a teaching experiment that supports collective argumentation in teaching triangles at the high school level and to present a teachers' participation in collective argumentation framework. The theoretical perspective adopted in this study is Krummheuer's (1995) collective argumentation perspective. The participants were 18 volunteer high school students and their mathematics teacher, who conducted the teaching experiment as a teacher-researcher. Data were obtained from video records of the collective argumentation process that took place in the teaching of triangles in 24 lessons over three weeks. Transcriptions of the video records and screenshots of students' work were analyzed first using a Toulmin argumentation diagram and then analyzed to determine teacher participation. The findings of the study presented the framework of teachers' participation in collective argumentation, which comprised six categories including teacher actions: (1) Presenting a direct argumentation component, (2) Encouraging students to present an argumentation component, (3) Moderating the argumentation, (4) Making clearer, (5) Actions in mathematical processes, and (6) Initiating and maintaining the norms that support argumentation. This framework contributes to the existing literature by focusing on both the teachers' participation and actions to support the collective argumentation process in long-term teaching.