Background and Aim: Medication error is one of the prevalent medical errors that most occurs among nursing students than any other group. The aim of this study was to determine the causes of medication error incidence from the viewpoint of nursing students.
 Materials and Methods: This study was a cross-sectional study that was conducted in 2015. The number of 137 undergraduate students in the third year of nursing course and higher of Khorramabad School of Nursing were selected using convenience sampling method. The data collection tool used in the study were demographic question and causes of medication errors. Descriptive statistics include percentage and mean used in the analysis of the data.
 Results: The results of the present study showed that based on subjects’ opinion the causes of medication error were respectively bad hand writing of medication orders in Kardex (65.9%), incorrect drug calculations (%63.8) and wrong writing drug in Kardex (%63). Additionally %35.5 of the students believed that medication without physician’s order was of the factors that lead to medication error.
 Conclusions: The results suggested carelessness to be the most prevalent cause of medication errors by students. Therefore it is suggested that nursing students pay more attention to reading patient records and also reading and writing patient kardex.