This study investigated the protective effect of the butanol (BuOH) fraction from Laminaria japonica (BFLJ) extract on high glucose-induced oxidative stress in human umbilical vein endothelial cells (HUVECs). Freeze-dried L japonica was extracted with distilled water, and the extracted solution was mixed with ethanol then centrifuged. The supernatant was subjected to sequential fractionation with various solvents. The BuOH fraction was used in this study because it possessed the strongest antioxidant activity among the various solvent fractions. To determine the protective effect of the BFLJ, oxidative stress was induced by exposing of HUVECs to the high glucose (30 mM) or normal glucose (5.5 mM) for 48 hr. Cell viability, lipid peroxidation, glutathione (GSH) concentration, and antioxidant enzyme activities such as catalase, superoxide dismutase (SOD), glutathione peroxidase (GSH-px), and glutathion reductase (GSH-re) were measured. Exposure of HUVECs to high glucose for 48 hr resulted in a significant (p<0.05) decrease in cell viability, SOD, GSH-px and GSH-re and a significant (p<0.05) increase in thiobarbituric acid reactive substances (TBARS) formation in comparison to the cells treated with 5.5 mM glucose or untreated with glucose. BFLJ treatment decreased TBARS formation and increased cell viability, GSH concentration, and activities of antioxidant enzymes including catalase, SOD, GSH-px, and GSH-re in high glucose pretreated HUVECs. These results suggest that BFLJ may be able to protect HUVECs from high glucose-induced oxidative stress, partially through the antioxidative defence systems.