Agency is regarded as the fundamental construct in second language (L2) learning. Despite the importance, agency has not been fully investigated in children's L2 learning. Moreover, there are predominant dichotomous views of agency, such as conformity and resistance. To gain an understanding of the complex concept of agency, this study asked two research questions: 1) How does an L2 child's agency emerge and change in social interaction over time in a preschool classroom? 2) How does co-evolution of the L2 child's agency, interaction, and participation of the L2 child influence his learning English as an L2? This research employed an ethnographic case study. During the 2015-2016 academic year, one Korean-American boy, Han, was observed in his church-based preschool classroom. Primary data sources included: a) observation audio records, b) field notes, c) three interviews with classroom teachers, Han's parent, and all 17 children, and d) photographed artifacts. After transcribing more than 80 hours of data, this study employed multiple data sources to answer each research question in association with multiple round coding cycles. This included a friendship network map from children's interview data and descriptive statistics of Han's English learning outcomes by using web-based lexical and syntactic complexity analyzer. In the findings, eight types of agency illustrated how Han identified his position relative to others, actively decided what role he would play to join a particular activity, and enacted his agency to represent himself in different ways. Han's English learning outcomes, including in the number of different vocabulary words, sentences, and verb phrases and the mean length of utterance, substantially improved over the year. By comparing English learning improvement to changes in the eight types of agency Han exhibited, this study was able to identify the strong relations between Han's English improvement and his agency of attempting to control interactions with socially unpowerful peers. This finding illuminated one possible way to be successful in L2 learning in which L2 children actively take the initiative in interactions rather than being prosocial and adopting follower roles. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://bibliotheek.ehb.be:2222/en-US/products/dissertations/individuals.shtml.]