PURPOSE: This study was conducted to examine the current status of transformational leadership behavior (TLB) and organizational citizenship behavior (OCB), and to investigate the mediating effect of social capital on the relationship between TLB and OCB in hospital nurses. METHODS: A cross-sectional descriptive study design was used with a convenience sample of 219 nurses from two university-affiliated teaching hospitals in South Korea. RESULTS: The survey instruments measured TLB, OCB, and social capital. Data were analyzed using t-tests, ANOVA, Scheffé's test, Baron and Kenny's regression method. The average level of TLB was 3.13 points, for OCB 3.64 points, and for social capital 3.24 points in social capital. Positive correlations were found between TLB, OCB, and social capital. Hospital nurses' social capital showed a significant mediating effect on the relationship between TLB and OCB. CONCLUSION: Findings indicate that TLB is a contributing factor to better OCB, and TLB can lead to improved OCB through social capital. Interventions to improve social capital of nurses in health-care organizations have important implications for OCB improvement.