This study is a case study that explores the experience of six professors(gender: female 2, male 4, major: Elementary Education, Ethics Education, Music Education, Physical Education 2, Practical Course Education) at the University of Education who train elementary school teachers participating in a lecture disclosure program to enhance their professionalism. Since the mid-2000s, to develop teaching professionalism a culture of observing the classes of fellow professors and contemplating such classes together has been maintained at the C University of Education. Since 2018, policy studies related to the class disclosure of professors have been conducted. Furthermore, in 2020, each of six professors opened each class at least once followed by an individual conference discussing opinions on class and implications for lecture improvement, and a post-mortem seminar in the form of a meeting with all research participants. In the same year, however, the majority of publicly available classes in universities were conducted online due to the COVID-19 pandemic. The process of observing the classes of fellow professors also occurred within the new platform, where the participants confirmed that various methods, such as discussion, practical, and project-based learning classes, were applied in online classes as well. Through the disclosure of classes among fellow professors, the participants were able to view their classes from an objective point of view, such that they were able to point out areas of improvement. Moreover, they comprehensively observed the educational process of pre-primary teachers through classes at the University of Education. The study expects that the results will contribute to the spread of the culture of open teaching in universities and draw implications for the self-professional development and improvement of professors of teacher education.