One of the basic fundamentals of nursing practice is drug administration which needs skills, technique and paying attention to clients. Drug administration errors can cause serious difficulties in nurse's work and make the patients exposed to preventable hazards. This study aimed to determine the prevalence and cause of common medication administration errors in nursing was conducted. Materials and Methods: This study is a descriptive cross-sectional study and the study samples were 120 nursing staff. Data collecting tool was a researcher made questionnaire, which distributed among the samples after appointing its validity and reliability. A total number of 120 completed questionnaires were collected from subjects and then analyzed by SPSS. 15 software. Descriptive statistics (Frequency, mean and standard deviation) were applied to the data. Results: The study findings showed that 86% of nurses had medication errors. The most common type of medication errors were including: wrong in giving the correct dosage, omission and wrong speed infusion. In investigating the causes of medication errors revealed that fatigue resulting from heavy workload, nursing shortage and nurses' pharmacology Knowledge deficit and illegible physician order is main causes of medication errors among nurses. Conclusion: According to of the findings of this study, all drug errors are preventable. Increasing the number of patients presence of ill patients have a role in occurring errors in drug administration, which propounds the necessity of conducting educational classes and annually pharmacological examinations for nurses and adjusting the proportion of nurses number to patients.