Lesson study is an approach to improve teaching and learning in the classroom through a particular form of activity where a group of teachers work collaboratively to design, implement, observe, and reflect on the proposed “research lessons” under the particular theme designated. While recent advocacy of lesson study is focused on its function of professional development of teachers, the cyclic nature of lesson study has a capacity to provide opportunities for developing innovative instructional materials. This chapter illustrates how lesson study can contribute to designing tasks and activities in the classroom and to developing an instructional sequence for teaching and learning of mathematical modelling. The case of public lesson study at a university-affiliated high school is used for illuminating how lesson study procced with task design and its implementation. The focus of this chapter is on the relationship between mathematical modelling and lesson study.