PURPOSE: This study discovered a substantive theory of the experience and process of peer support work among people with mental illness. METHODS: The participants were members of community-based mental health facilities and had been working as peer supporters for more than six months. The data were collected through in-depth interviews with twelve participants and analyzed using Corbin and Strauss's grounded theory approach. RESULTS: The core category was "becoming a healer going with patients in the journey of recovery," and the core phenomenon was "identity confusion as a peer supporter." The causal conditions were "starting peer support work without certainty" and "standing at the boundary between the therapist and patient." The intervening conditions were "willingness to become a successful peer supporter," "feeling a sense of homogeneity with the patient," "accepting the mental illness," and "support from people around." The action and interaction strategies were "letting go of greed," "being open about oneself," "developing professional skills," "maintaining wellness in the body and mind," and "being with the patient." The consequences were "becoming a useful person," "changing attitude toward life," "expansion of the sense of self-existence," "recovering from mental illness," and "discovering a role as peer supporter." Finally, the substantive theory of "becoming a healer going with patients in the journey of recovery" was derived. CONCLUSION: This study provides a holistic understanding of peer support work and the implications of interventions to help people with mental illness in a person-centered recovery process.