The aim of this paper is to propose principles for and ways of using ChatGPT in writing education for students in the discipline in which they are majoring. The analysis target is a learner’s writing task on “The Results and Implications of Using ChatGPT” calculated from “the Korean phonology” of the University of Education. First, the educational implications of the task were defined in detail and shared with learners. The actual usage method of learners and the expansion process were then analyzed. Instructors and learners in the subject also discussed whether ChatGPT can be used to develop thinking skills for writing in their discipline. I note that the goal of using ChatGPT for major writing is essentially subordinate to the goal of major writing education, which internalizes the knowledge structures and creates meaningful knowledge. I also note that learning the structures of knowledge means learning ‘ways of acting,’ ‘ways of knowing,’ and ‘ways of thinking,’ which are applied to major writing education using ChatGPT. In addition, I discuss the importance of education that leads learners to develop their thinking skills by recognizing the content of their perception of the use of ChatGPT as ‘realized’ and using it as an opportunity to discover the ‘potential’ it implies. I also emphasize the importance of finding ways to expand the system that can enhance this thinking ability.