The purpose of the study was to investigate how Lesson Study influences pre-service teachers' learning to professionally notice students' mathematical thinking. The initial and last video-recorded discussions were analysed using a qualitative method. Findings suggest that Lesson Study can be useful means to improve pre-service teachers' noticing expertise in reviewing and planning lessons. However, to maximise the effect of Lesson Study on developing pre-service teachers' noticing expertise, several modifications need to be made to the Lesson Study protocol, which are discussed.