This study aims at exploring how to build an effective online classes by examining the Virtual Summer Intensive Program (VSI) hosted by the National Security Language Initiative for Youth (NSLI-Y) in Jeonju, South Korea. The analysis was conducted in two parts; one is the preparation of an online class and the other is the administration of the class. First, the class preparation is subdivided into the following phases: restructuring the curriculum, revising syllabus and adapting to digital devices. The curriculum restructuring requires changes of the following: development of prerequisite learning materials, roles of teaching assistants and tutors, and diversification of culture activities. The managers of the VSI program revised the course curriculum and held meetings to reduce technical difficulties. Second, the class administration was examined with the factors of teachers, learners, and technology. The teachers maximized interactions to make students engaged in the class and motivated students to work outside of class by encouraging them to work with teacher’s assistants. The students were informed of the online classroom guidelines and asked to comply with them. The technology enabled to record online lectures, utilize online resources, and participate in online culture activities. To assess the effectiveness of online teaching, the students’ language skills were measured through the Oral Proficiency Interview (OPI). The OPI test results revealed that all of the VSI participants demonstrated a significant increase in their language skills, which proved the effectiveness of the VSI classes.