A composition for prevention or treatment of metabolic syndrome or for antioxidation includes black soybean leaf extracts and flavonol glycoside compounds isolated thereof as active ingredients. It has been confirmed that the black soybean leaf extracts and the flavonol glycoside compounds isolated thereof effectively inhibit α-glucosidase activity; LDL oxidation and DDP-4; have excellent DPPH radical scavenging activity; inhibitory activity of reactive oxygen species (ROS) accumulation; effectively promote insulin secretion; regulate the expression of major genes; have an excellent ability to reduce a waist/hip circumference ratio; decrease the concentrations of plasma free fatty acid and plasma triglyceride; increase the concentration of plasma HDL-cholesterol; decrease an atherogenic index; increase erythrocyte catalase, SOD and GR activities; decrease erythrocyte lipid peroxide TBARS; decrease the concentrations of inflammatory cytokines MCP-1 and PAI-1, and resistin; and increase the concentration of plasma adiponectin.