This article studies the formation of the identity of an in-service mathematics teacher who simultaneously enters a process of professionalization in relation to her teaching activity. It is a case study. Theoretically, the structuring of identity is framed within a social perspective of learning, called communities of practice. Evidence of professional identity was methodologically collected through interviews, professional narratives, classroom observations and teacher productions. Evidence shows that her identity traits are associated with the vision of what she assumes teaching activity provides, professionally, and what she imagines it to be; as well as with the acquisition of knowledge that transcends discipline.