In this study, we examine how 30 pre-service teachers designed and modified mathematical tasks to enhance primary students’ learning. The pre-service teachers took part in a 5-week course in a teacher education program in Sweden in which a theory-based lesson study model entitled learning study were established to deepen the teachers’ awareness about the relationship between instruction and student learning. The learning study course design consisted of two intervention cycles in which the pre- 17 SIG 9 Phenomenography and Variation Theory service used variation theory as a tool for lesson design and re-design. The aim of this study is to explore in what ways the pre-service teachers modify mathematical tasks when employing variation theory during the 5-week mathematics education course in which LS cycles were incorporated. Data were collected at the end of the course and consist of written reports about task refinements based on their reflections about the students’ performance during the lessons. We identified five different ways of task modifications: expanding tasks, making tasks more explicit, making tasks more implicit, bringing metaphors and representations to the foreground, and creating new tasks. These categories might become a complementary tool to variation theory, for reflecting about task design and redesign.