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). Result of inflammatory marker (hs-CRP), oxidative markers (NOx and malondialdehyde) will be presented. Conclusion This study indicated that 3-night shift aggravated endothelial function. However, consumption of small amount of polyphenol-rich dark chocolate may improve the endothelial function.