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Abstract Background: Misconceptions of mental models are phenomena that are variance with accepted scientific models. Purpose of this study was to investigate the prevalence of misconception about half life and concentration of insulin hormone in students of Arak university of medical sciences. Materials and Methods: In this descriptive study 153 undergraduates in medicine and nursing – midwifery faculties of Arak university of medical sciences in 2009, were asked to answer to two questions about half life and concentration of insulin hormone. Results: The prevalence of misconception about half life of insulin hormone in medical, nursing and midwifery students were 75%, 80% and 85%, respectively, and prevalence of misconception about insulin concentration in blood were 31%, 26% and 20%, respectively. Conclusion: Misconceptions about insulin half life and concentration exist in high percent of students and it is necessary to correct the teaching pattern.