With solvent extraction, a total of four fractions were separated from 75% alcohol extract of Monochoria vaginalis, and their antioxidative activities were measured by iodine method. The results showed that among the fractions, n-butanol fraction exhibited the highest antioxidative activity, which was not only significantly higher than CK (water), but also equivalent to the natural antioxidant tea polyphenols and synthetic antioxidant butylated hydroxyl toluene (BHT). A compound was isolated from the n-butanol fraction by using column chromatography, and identified as stigmasterol 3-O-beta-D-glucopyranoside. Its antioxidative activity estimated by the determination of the percentage of scavenged free radicals indicated that this compound exhibited a higher activity than BHT in scavenging free radicals.