Purpose: This paper presented the learning process of a group of primary mathematics teachers who participated in two iterations of lesson design, enactment and reflection in a Chinese Lesson Study. Design/methodology/approach: An expansive learning theory was employed to examine the teachers' learning process in lesson study (LS) on representing fractions on a number line. The evolution of a germ cell was utilized to feature the transformation of the object of activity from abstract to concrete through resolving contradictions among LS members. The videos of lesson planning, research lessons (RLs) and debriefing meetings were collected and analyzed to reveal the expansive learning process. Findings: The analysis showed that the teachers expanded their learning through transforming the object from diffuse to concrete and expanded through consciously articulating the germ cell. The outcomes of object-oriented activity include improving the enacted lesson which promoted students' conceptual understanding. Originality/value: This study made a unique contribution to understanding the learning process of teachers in Chinese LS from the perspective of expansive learning.