In the course of screening for antioxidant compounds by measuring the DPPH (1,1-diphenyl-2-picrylhydrazyl) radical scavenging effect and superoxide quenching activity, methanol extract of Cyperus difformis (Cyperaceae) was found to show potent antioxidant activities. Subsequent activity-guided fractionation of the methanolic extract led to the isolation of three phenolic compounds, rosmarinic acid (1), luteolin (2) and caffeic acid (3). Their structures were elucidated by spectroscopic studies. Three compounds showed the significant antioxidative effects on DPPH and nitric oxide radical scavenging effects, riboflavin and xanthin originated superoxide quenching activities. Among them, compound 3 showed the most significant antioxidative effects on those four antioxidant tests. Compound 3 showed better antioxidative activities than vitamin C.