Introduction: Feedback cards are recommended as a feasible toolfor structured written feedback delivery in clinical education whileeffectiveness of this tool on the medical students’ performanceis still questionable. The purpose of this study was to comparethe effects of structured written feedback by cards as well asverbal feedback versus verbal feedback alone on the clinicalperformance of medical students at the Mini Clinical EvaluationExercise (Mini-CEX) test in an outpatient clinic.Methods: This is a quasi-experimental study with pre- and posttestcomprising four groups in two terms of medical students’externship. The students’ performance was assessed throughthe Mini-Clinical Evaluation Exercise (Mini-CEX) as a clinicalperformance evaluation tool. Structured written feedbacks weregiven to two experimental groups by designed feedback cards aswell as verbal feedback, while in the two control groups feedbackwas delivered verbally as a routine approach in clinical education.Results: By consecutive sampling method, 62 externship studentswere enrolled in this study and seven students were excluded fromthe final analysis due to their absence for three days. Accordingto the ANOVA analysis and Post Hoc Tukey test, no statisticallysignificant difference was observed among the four groups atthe pre-test, whereas a statistically significant difference wasobserved between the experimental and control groups at thepost-test (F=4.023, p=0.012). The effect size of the structuredwritten feedbacks on clinical performance was 0.19.Conclusion: Structured written feedback by cards could improvethe performance of medical students in a statistical sense. Furtherstudies must be conducted in other clinical courses with longerdurations.Keywords: Feedback; Medical education; Ambulatory care; Outpatient clinics