Background/purpose: Education is crucial for fostering sustainability awareness and thinking among university students in a world increasingly defined by mounting environmental crises. Textbooks, as fundamental learning resources, have the potential to shape the minds of global citizens by offering insights into contemporary environmental challenges. This study aimed to investigate the promotion of sustainable thinking in international English language textbooks by incorporating Green Education elements throughout various aspects of the Unlock English series. Materials/methods: In light of the research objectives, a series of textbooks (A1, A2, B1, and B2 levels) from the Unlock series has been analyzed using content analysis and criteria developed by the researchers, which covered various dimensions of Green Education. Results: The findings revealed that the textbooks moderately included Green Education components, primarily focusing on increasing students' environmental awareness. However, there was a noticeable lack of content aimed at developing personal environmental values, critical thinking skills, and active participation in action-oriented activities. In contrast, the textbooks were found to have the potential to enhance sustainability thinking among university students. Conclusion: The study concluded that there is a need for a more balanced inclusion of Green Education components across textbooks of various language proficiency levels to promote holistic, sustainable thinking.