Background and Aim: According to the fact that Critical thinking is a fundamental qualification for students in all fields of academia, the aim of this study was to investigate critical thinking skills of first- to fourth-year residents in the Medical School of Mashhad University of Medical Sciences.

Materials and Methods: This is an empirical and cross-sectional study. The study population consisted of 387 residents who were enrolled in 2 fields and 11 sub-fields of the medical school during the academic year 2014-2015. By Using the stratified random sampling method a sample of 156 residents was selected. Demographic data forms and questionnaire related to the California Critical Thinking Skills test were used for the purpose of data collection. For data analysis one-sample t-test, t-test for independent groups, and one-way ANOVA were used.

Results: The analysis showed that the critical thinking of students is lower than the optimal level (p <0.001) and there is no significant difference among residents of the four categories (first-to fourth-year) in terms of their critical thinking abilities. 

Conclusion: Overall, the results show that residents do not perform well in terms of possession of critical thinking skills. Accordingly, it is recommended that medical schools should pay more attention to the fostering of critical thinking abilities, as well as, active learning and teaching approaches.