This paper presents a study of primary school teachers’ practices in mathematics in the context of a professional development training, lesson study (LS) in Lausanne (Switzerland). The study deals with the following question: what practices will change and what resistance in practices will be observed when primary school teachers engage in LS as a form of professional development? The practices are analyzed in a double didactical and ergonomical approach theoretical framework and the qualitative method used is a case study of one particular teacher. This study underlines the evolution of the teacher’s practices concerning the choice of mathematical activities and the interactions between the teacher and the students. The study highlights some limits linked to the difficulty to reinvest and to transfer professional acts, didactical and mathematical knowledge worked during the lesson study process.