Introduction: Medication error is one of the quality problems in hospitals harming millions of people around the world every year. This study aimed to investigate the occurrence and reporting of medication errors by nurses in hospitals affiliated with Mashhad University of Medical Sciences, Mashhad, Iran. Materials and Methods: This descriptive cross-sectional study was conducted on 530 nurses selected by simple and stratified random sampling in 2014. Data were collected using a survey consisting of four sections and 66 questions, scored based on a Likert scale (87% return rate). Data analysis was performed in SPSS Version 18 using descriptive statistics, ANOVA test, and chi-square test. P-value of less than 0.05 was considered statistically significance. Results: The most prevalent medication errors by nurses was early or late administration of medication (43.7%), which was attributed to individual factors by the managers in the viewpoint of the nurses (mean: 3.66+1.3). In addition, the occurrence of medication error might be due to the hospital ward patient overcrowding (4.29+1.07). No significant relationship was observed between the variables of medication error, the causes, and lack of error report with gender and nursing experience (P=0.6 and P=0.8, respectively). Conclusion: One of the effective methods for prevention of medication error is providing teaching courses for nurses to raise their knowledge in this regard and aware them of the outcomes of wrong medication prescription.