As great attention is given to a high quality of English education in Korea, more and more in-service and pre-service English teachers are looking for an opportunity to study in an English speaking country to become better qualified teachers. However, after receiving a degree in an English speaking country, many teachers fail to apply what they have learned to their own teaching due to their tensions of identity, beliefs, knowledge, and professionalism within the changes of sociocultural settings. By using sociocultural theory as a theoretical framework, this paper explores how formal training and Ph.D. studies in the U.S. have influenced a Korean teacher educator in applying theory to practice in relation to her identity, beliefs, knowledge, and professionalism during 30 years of her teaching experience. Rather than facing tensions, the teacher educator has been willing to change her roles, broaden and deepen her beliefs in teaching and knowledge about theory of teaching and learning, and continue her professional development. Limitations and implications of the study are provided.