Teacher identity has been an important issue in teacher education because teacher identity influences teachers' professional development. However, little has been explored in preservice teachers' identity formation within the EFL context of language teaching. In this study, the early influence on EFL student teachers' identity formation in practicums was studied from the perspective of legitimate peripheral participation (Lave & Wenger, 1991). Ten participants enrolled in the practicum courses of the four educational institutions, organized by the English department of a national university in northern Taiwan. The frameworks of the practicums at each school were analyzed and the results for each case study revealed contextual factors that support and weaken teachers' professional identities. Three features were identified in the student teachers' identity formation: (1) a hybrid space between formal teachers and student teachers, (2) adhering to the institutions' demands-progressing from the periphery to the center, and (3) struggling teacher identity. Based on the findings, relevant pedagogical implications are discussed to help L2 preservice teachers achieve success in practicums.