Critical thinking (CT) is an essential higher-order thinking skill for students. Although numerous Taiwanese studies have focused on the teaching of CT and its association with English , few have investigated its representation in English textbooks. In the present study, content analysis of CT representation in Taiwan’s senior high school English textbooks was performed to determine the extent to which secondary school English textbooks support CT teaching. The 593 questions in the reading sections of the L Book series were used for a case study. Drawing on Bloom’s taxonomy and Taiwan’s English curriculum guidelines, the researcher developed a coding scheme comprising four CT categories—analysis, evaluation, synthesis, and creation —and analyzed how CT was presented within and across textbook volumes. The results indicated that one-third of the questions in the L Book series could enhance students’ CT skills, with analysis and evaluation being the most prevalent categories. Furthermore, a progressive shift from analysis and evaluation to creation was observed across volumes, indicating a cumulative hierarchy in CT development. These findings offer pedagogical insights for senior high school English teachers and textbook publishers regarding the cultivation of CT skills in students and the improvement of English textbook design in Taiwan.