In the past two decades, different aspects of teacher identity have been widely studied and discussed. However, in the field of English language teaching, the issue of identity among English for Specific Purposes (ESP) teachers seems to be an under-researched topic. This study focused on the narratives of a teacher to gain insight into her identity development during the process of becoming an ESP instructor. Data collection tools included initial reflection (before ESP instruction), learning journal, teaching journal, face-to-face meeting, and follow-up interview. Using the categorical analysis method, the study broke the collected texts into smaller units for descriptive treatment and examined some linguistic characteristics of this participant. The findings indicated an ESP teacher has to be prepared for extensive learning, be engaged in interdisciplinary collaboration, meet the expectations of trouble-shooting for a variety of problems, and undertake multiple tasks during lesson delivery. Most importantly, from EAP to ESP teaching, a teacher faces constant adjustments in teaching approach, lesson pace, and class activity. Therefore, new ESP teachers should be prepared for potential challenges in reconciling identity conflicts for professional growth.