This study intends to identify some key factors in creating and sustaining school-based teacher professional learning communities (PLCs) through a case study of a South Korean public high school. To achieve this, the study identified some essential infrastructure, preparation, and necessary social organization for creating PLCs. The ideal unit and the encouraging/discouraging factors in the implementation process were also investigated. Data were gathered via classroom observations and by analysis of interview transcripts, questionnaire responses, and minutes from PLC meetings. Nineteen participants, including 16 teachers, a principal, an assistant principal, and a facilitator from a city department of education, who assisted the school reform process, completed the questionnaires. Three of the teachers who took the reform initiative participated in the in-depth interview. The study provides a detailed description of the school context before the PLC implementation, challenges that faced the teachers, and two main characteristics of their PLC initiative. The study indicates that participants perceived prepared teacher leaders, building trust and respect among faculty, and securing time for classroom observation and PLC meetings as the most necessary preparation in creating and implementing their PLC. Empowering grade level chairs, increasing teacher proximity, and employing additional administrative assistants were identified as effective administrative support. Participants recognized that each grade level had more advantage in implementing PLCs and thought positive changes of disruptive students and their own instructional practices were the most encouraging factors in overcoming implementation problems. Authoritative leadership of school administration and a city DOE that forcefully mandates PLCs were perceived as discouraging factors in PLC implementation.