Since its release in November 2022, ChatGPT has brought both opportunities and challenges to English education. While it enhances writing and engagement, concerns remain about over-reliance and ethical use. Metacognition is key in language learning, yet little is known about how ChatGPT affects this in advanced Japanese university students. This eight-week study in 2024 involved 30 students who received structured guidance on AI use, conducted research projects, recorded their interactions with ChatGPT, and reflected on their learning. It focused on three questions: (1) How does explicit guidance affect metacognitive awareness? (2) What support fosters metacognitive skills? (3) How do AI usage patterns show learner autonomy? Qualitative analysis revealed that motivated students used ChatGPT to refine their thinking, while others struggled with over-reliance. The study found that AI can support brainstorming but may hinder self-reflection without careful guidance. This presentation highlights the importance of instructional support to help students use AI intentionally. By encouraging reflective thinking and purposeful interaction with ChatGPT, teachers can promote both language development and metacognitive growth.