Background and purpose: It is now clearly acknowledged that unjustified prescription habit can lead to ineffective or incomplete treatment and may extend the course of illness or time to recovery. At present, there is no taught course available on prescription writing method specifically for common illnesses. Consequently, students learn through experience. In these circumstances, it seems essential to teach medical students the prescription techniques and insight into national pharmacology. Therefore, this study aims to evaluate diagnosis, treatment and prescription writing capability of graduating interns for common clinical scenarios.Materials and methods: This was a cross sectional study assessing the prescription writing ability of 41 interns for 10 common clinical situations based on an Objective Structured Clinical Examination (OSCE).Results: In every station, average action was 1.57. From this, 68% were correct performance. Average number of medicine for each prescription was 1.8, with 70% appropriate medicine. From correct prescriptions, 59% had accurate dosage and 44% with precise course of treatment. On average, there were 1.4 right and 0.29 wrong medical advices per prescription. Over all, 82.7% of recommendations were correct.Average score of interns in all stations was 56.4%. There was a correlation between prescription writing ability and the average mark of interns. More than half of the interns had average ability in writing prescription and there was no relation between their ability and age, sex, marital status, or their medical exam (G.P.A) score at pre-internship exam.Conclusion: Generally more than a third of prescriptions written by students graduated from Sari Medical School were wrong and only 5% of them were absolutely accurate. Conducting practical educational programs of pharmacology, as WHO pattern, seems reasonable.