Medication errors are among the most common medical errors in the hospitals. Transcription error is a specific type of medication errors and is due to data entry error that is commonly made by the human operators. This study was designed to detect transcription errors in a teaching hospital in Tehran. Direct observational method was used in this study. Error was defined as any deviation in transcribing medication order from the previous step (order on the order sheet, administration nursing note and/or cardex, documentation of the order in the pharmacy database). A total of 287 charts with 558 opportunities for error were reviewed. Of those opportunities for error 167 (29.9%) resulted in an error. Omission (the patient did not receive the medication that was ordered) was the highest (52%) transcription error type seen in this study. The evaluation clearly showed that errors at transcription stage were not infrequent. To cut these errors down we suggest implementation of surveillance systems, which might help to decrease medication errors.