Purpose: This study was done to identify determinants of registered nurse (RN) skill mix and staffing level focused on hospital characteristics. Methods: Data were obtained from health insurance claims data and hospital reporting system in the Health Insurance Review and Assessment Service (HIRA) for the year 2010. Data from 2,998 hospitals were analysed using t-test, ANOVA, Pearson correlation, and regression analysis. Results: The RN skill mix and staffing level were positively related to hospital size and the percentage of inpatients to total patients. RN skill mix and staffing level were statistically different across regions. Including nursing aides (NA), however, there was no difference in staffing levels across regions. Medically vulnerable regions, bed operation rate, and the number of patients per doctor were also related to RN skill mix and staffing level. Conclusion: The statically significant determinants of RN skill mix and staffing level included hospital size, region, bed operation rate, percentage of inpatients, doctor-patient ratio. Further study needs to be done to investigate factors including RN supply and wages.