The purpose of this study is to explore how a teacher adapts the textbookactivity on polyhedron and what factors affect the adaptation. The data of thisstudy was collected from a primary school teacher, also an experienced textbookeditor, who has been teaching for twenty years; the method used includesclassroom observation, in-depth interview, and focus group discussion. Theteacher uses four types of textbook adaptation for activities and questions whenshe teaches students about polyhedron: addition, replacement, omission, andreorganization. Three factors may affect the adaptation. The first one is therepresentation of the textbook, which includes the physical objects, tasks, andconcepts. The second factor is teacher’s personal knowledge and habit whichincludes mathematics knowledge, knowledge of students’ mathematical learning,mathematics pedagogical content knowledge, and personal habit and perspective.The last one is the context which includes time, quantity of teaching aids, andsupport from the experts. Last but not the least, this study makes suggestions onthe design of the textbook, teacher training and important aspects of further study.