Background : Previous our study suggests that three sequential night shifts deteriorated endothelial function through decreased production of NO. Flavanols have been suggested to mediate the favorable effects on cardiovascular health. The aim of this study is to examine the effect of flavanol-rich cocoa on endothelial function and oxidative stress in healthy medical personnel with three sequential night shifts. Method : This Prospective, open-label, case-control study was designed for forty young healthy nurses divided into two groups. The cocoa group (n=20, 28.5짹4.5 years) received dark chocholate 48g/day (containing 813mg polyphenols) for 3 days and control group (n=20, 27.4짹3.2 years) did not received chocholate. We examined Flow-Mediated Dilatation (FMD), hs-CRP, oxidative markers (Nitric oxide and malondialdehyde) before and after three-night shift. Results : Baseline characteristics were not different between the two groups. FMD was significantly decreased after continuous 3-night shift in both two groups (p<0.001). FMD change which means deterioration of vascular endothelial function was lower in cocoa group than control group (5.83% vs 3.14%, p=0.041). There was no significant change in hs-CRP after 3 sequential night shifts in both groups (0.4짹0.7 mg/L vs 0.8짹0.8 mg/L, p=0.78). There was also no significant change in MDA. However, in cocoa group, NOx was more increased than control group after 3 sequential night shifts (172.0짹64.1 mmol/dL vs 136.8짹66.2 mmol/dL, p=0.023). Conclusion : This study indicated that 3-night shift aggravated endothelial function. However, consumption of small amount of polyphenol-rich dark chocolate may improve the vascular endothelial function.