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The current study uses the Rasch Model to measure lower-secondary school students' critical thinking skills on photosynthesis topics. Critical thinking skills are considered essential in science education, but few valid and practical measurement instruments remain. The current study fills the gap by adapting the instrument from the Watson-Glaser Critical Thinking Appraisal. A quantitative approach with a survey design was used in the study, involving 467 lower-secondary school students in Klaten, Central Java, Indonesia. Data were analyzed using the Rasch Model to evaluate each item's reliability, validity, level of difficulty, and discrimination ability. The results suggest that the instrument has good validity and reliability, with the inference subscale being the most difficult, while evaluation is the easiest for students to master. The findings provide insights for educators in designing more effective teaching strategies to develop critical thinking skills. The study implies that a valid measurement instrument is obtained and offers new insights into the profile of students' critical thinking skills in science learning to support more effective science teaching in secondary schools, especially on photosynthesis.