This study examines the implementation of Written Corrective Feedback (WCF) in undergraduate EFL students’ thesis writing at a university in Surakarta. Using a qualitative case study approach, it explores the types and strategies of WCF and students’ perceptions of its impact on their revisions. Findings show that WCF helps students improve grammar, structure, and academic writing. Still, some struggle with unclear feedback, leading to confusion. Students receive direct focused feedback (explicit corrections), indirect focused feedback (highlighting errors), and indirect unfocused feedback (addressing multiple errors). Direct feedback is the most effective, while indirect feedback encourages self-correction but may require clarification. Online feedback also presents challenges due to limited explanations. Analyzed through behaviorist theory and Ellis’s (2009) WCF framework, this study highlights the importance of structured feedback and suggests combining direct and indirect strategies to enhance thesis writing. Future research should explore WCF’s long-term impact and the role of technology in facilitating feedback application.