Introduction: In the education of medical sciences-e g, nursing, the main goal is to train students who can make the best decisions with high thinking skills (critical thinking), integrating information, and using sound clinical reasoning while relying on evidence.Objectives: The current study aimed at investigating the effect of teaching clinical reasoning on the critical thinking skill of undergraduate nursing students who took the course of emergency nursing in disasters and events. Materials and Methods: The current quasi-experimental study was conducted on 27 undergraduate students of the 6th semester in Aja Nursing Faculty enrolled in a full-scale study in 2018. The critical thinking styles of students were assessed using the California critical thinking questionnaire before and two weeks after training. The clinical reasoning training program was presented using a repeat hypothesis model in five educational sessions. Data were analyzed by SPSS software version 23 using paired samples t-test.Results: In the present study, the mean and standard deviation scores of critical thinking were respectively 7.63 ± 2.02 and 7.9 ± 3.36 before and after the training program. There was a significant difference between before and after mean scores of critical thinking education (P = 0.047); training with clinical reasoning method improved the critical thinking skills compared to the study onset in the students. Discussion and Conclusion:The critical thinking level of nursing students increased significantly after receiving educations with the method of clinical reasoning compared to the study onset level. It can be concluded that this educational method, like other active teaching methods, can improve critical thinking in nursing students.It is hoped that using this method will be an effective step toward better learning of the students and improving the performance of nurses in clinical practice.