To examine the relationship between nurse staffing and quality of care of nursing home residents, longitudinally. Mixed results abound on the relationship between nurse staffing and quality of care of residents in nursing homes. Cross-sectional designs may underpin bias because the relationship between staffing and quality emerges over time, with latent unobserved variables. To offset this limitation, I used a longitudinal design in this study. I used repeated quarterly survey methods. I measured staffing information with a formula developed by the Centers for Medicare & Medicaid Services. Outcome variables were 15 quality indicators and the Korean National Health Insurance Service 2015 nursing home evaluation manual. As registered nurse hours per resident day increased by 1, the percentage of residents with depression decreased by 3.88%, and the prevalence of residents with nasogastric tubes increased by 1.17% in 3months. Prevalence of residents with bed rest decreased by 5.72%, and residents with restraints decreased by 1.092%. More registered nurses and fewer certified nursing assistants or qualified care workers yielded a statistically significant negative influence on aggressive behavior, depression, weight loss, and bed rest. The higher turnover of total nursing staff related to more use of antidepressants. Results supported registered nurses' exclusive impact on resident outcomes. More longitudinal research is required to confirm the influence of nurses on nursing home residents' outcomes. This study supported about the contributions of increased input of Registered Nurses, additional to previous longitudinal studies. The nursing homes in Korea should have mandatory Registered Nurse placement for optimal quality of care. This study supported about the contributions of increased input of Registered Nurses, additional to previous longitudinal studies. The nursing homes in Korea should have mandatory Registered Nurse placement for optimal quality of care.