Background & Objective: Critical thinking (CT) is essential in medical education and improves physicians' clinical ability and competency. The present study aimed to determine the CT level and the trend of its changes in medical education at Ahvaz Jundishapur University of Medical Sciences, Ahvaz, Iran, in 2021. Materials & Methods: In this cross-sectional descriptive study, the CT levels of 165 medical students in three curricular phases, namely basic sciences, externship, and internship courses were examined. The data collection tool was the California Critical Thinking Skills Questionnaire Form B. Data were analyzed by SPSS (version 20) using t-test, ANOVA, and Pearson correlation coefficient. Results: The mean score of medical students’,CT was 12. 07 ±,4. 48 out of a total score of 34. The mean scores of students’,CT in basic sciences, externship, and internship courses were 12. 75 ±,4. 47, 12. 82 ±,5. 10, and 10. 67 ±,3. 49, respectively. There was a significant difference between students' mean scores of CT in different curricular phases (P=0. 010). In addition, there was a significant difference between the mean scores of students' CT based on gender, and the mean scores of CT of male students were significantly higher (P=0. 049). Conclusion: The mean scores of CT of medical students were low and did not improve during their university years. Since CT plays an essential role in clinical decision-making, it is necessary for medical teachers and university officials to pay more attention to the medical students' CT development.