This paper presents a Learning Study case on the topic of weight in the subject of Mathematics in a Hong Kong school of students with mild intellectual disabilities. The research team, composed of teachers from four Special Education schools, academic staff from a University and officials from the Education Bureau. The three parties worked as a learning community to design and implement a research lesson on the concept of weight.In Special Education Schools, the most common tool for teaching the topic of weight is a set of balance scale to compare the weight of two different objects. Through the demonstration of comparing two different objects, the lower side of the balance scale indicates the heavier weight. The findings of the pre-lesson student interviews show that the students only memorized the visual image of the lower or higher level of the balance scale without understanding what makes the difference in the height level of the balance scale. After identifying the learning difficulties of the students, the teachers realized that the conceptual understanding of weight is far more important than utterance of the observable phenomenon of the balance scale.The lesson adopted a communicative approach using the Variation Theory as a pedagogical tool and integrating the STEM elements in designing the learning activities to help students discern the critical features of the object of learning, as well as explore the concept of weight comprehensively through observation and inquiry. Students were encouraged to use their innate ability to estimate the weight of some objects in their daily life even without a balance scale in hand.This Learning Study case reports the implementation process with the support of video clips of student interviews and the research lesson. It is hoped that this presentation will open up the discussion about teaching the topic of weight in the field of special education. Copyright © 2018 WALS.