This paper, which is one of the few in the world dealing with this topic, presented the psychologist's work based on the concepts of mentalization and internal working models (IWMs) of attachment, with an adolescent girl who was prone to nonsuicidal self-injury. Gaga, a student in the first grade of high school, had visible scars from cutting her hands on the inner sides of both forearms. Gaga told the psychologist that her mother believes that this is an attempt to draw attention to her problems, and even a suicide attempt. Together with Gaga, the psychologist made a formulation of her mental difficulties. The main aim of the psychologist's work with Gaga was to control unpleasant impulses and emotions, so as to eliminate self-injury. This aim has been achieved through three global tasks: 1) that Gaga, in the relationship with the psychologist, builds her IWMs as positive, so that she can see herself as a person who is worthy of the love and support of close people; 2) to develop a stable capacity for mentalization; 3) to apply this acquired skills in the interpretation of her affective experiences. The mentalizing approach in working with Gaga was based on an attitude of emotional warmth, acceptance and authentic interest in her inner world. The psychologist's work with Gaga led to her better self-control, which resulted in the cessation of her self-injury. In her work with a psychologist, Gaga acquired positive IWMs, which she transferred into other significant interpersonal relationships.