<p>This study examines the interaction patterns and perceptions of university students who used ChatGPT during the English essay writing process. Twenty university students in Korea were divided into two groups based on English proficiency: 10 advanced learners and 10 intermediate learners. Participants completed essay writing with the assistance of ChatGPT, and their interaction logs were collected throughout the process. The logs were analyzed using Epistemic Network Analysis (ENA) to identify interaction patterns across proficiency levels. In addition, thematic analysis of interview data was conducted to examine learners' perceptions of ChatGPT-supported writing. ENA results showed that advanced learners used ChatGPT primarily during the prewriting stage for information gathering, word choice exploration, and translation, and later shifted to refining expressions during drafting. In contrast, intermediate learners relied more heavily on ChatGPT during drafting, particularly for lexical and syntactic exploration. Thematic analysis revealed that although learners recognized ChatGPT as an efficient and supportive tool for English essay writing, they also noted its limitations, including unclear information sources and insufficient contextual understanding.</p>