Antioxidant active compounds were isolated from methanol crude extracts of the leaves of Guava (Psidium guajava L.) grown in Thailand. The isolated compounds were screened for their in vitro antioxidant activity by a DPPH free radical scavenging assay. Results indicate that three isolated compounds contribute importantly to the antioxidant activity of guava leaves, providing a scientific basis for the use of this plant in traditional medicine. Their structures were determined on the basis of spectroscopic and chemical methods. The most active compound was found to be quercetin along with two flavonoid compounds, quercetin-3-O-glucopyranoside and morin. The isolated quercetin, quercetin-3-O-glucopyranoside and morin showed significant scavenging activity with IC50 of 1.20±0.02, 3.58±0.05 and 5.41±0.20 μg/ml, respectively. © Österreichische Apotheker- Verlagsgesellschaft m. b. H.