Aim and background: Diagnostic thinking is the ultimate goal of educational system and the basis for clinical reasoning. The aim of this study was to assess the clinical reasoning and diagnostic thinking ability of dental students by key features test and Diagnostic thinking inventory (DTI) questionnaire. Materials and methods The present study was a descriptive cross-sectional study. The participants consisted of 61 senior dental students. Clinical reasoning and diagnostic thinking were assessed by key feature test and DTI questionnaire, respectively. To design the KF test questions, the blueprint of exam was first designed in expert panel based on dental curriculum. The questions developed based on common cases in oral and maxillofacial diseases by the group of oral and maxillofacial specialists. The DTI was developed by Bourdieu et al. in France and consists of 41 questions on a 6-point Likert scale, of which 21 are memory structure category and 20 are in flexibility in thinking category. Satisfaction of student assessed through a 10-item questionnaire. Data were analyzed using SPSS 19 by descriptive tests (mean, SD, percentage) and student independent T-test and Pearson test. Significance level was determined p < 0.05. Results The mean scores of the key features test of students were 56.55 ± 7.80. No significant difference was reported between clinical reasoning scores of key features test by students' gender (p-value = 0.19). There was no significant difference between the scores of diagnostic thinking between men and women (p-value = 0.11). The difference in students' scores in the domain of flexibility in thinking was significantly higher among male students than female students. (P-value = 0.04). There was no significant correlation between students' diagnostic thinking scores and their clinical reasoning scores in the key features test. Conclusion Based on the present results, the clinical reasoning and diagnostic thinking skills of participants were reported in the low level. This issue emphasizes the need for training to enhance diagnostic thinking and clinical reasoning in dental education. Formative evaluation and reform the educational programs of this course should be considered.

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