Phytochemicals contained in dietary plants provide a variety of health benefits and may reduce the risk of cardiovascular diseases. The aqueous extracts from three popular Thai dietary and herbal plants, Cratoxylum formosum, Syzygium gratum, and Limnophila aromatica, were investigated for the antioxidant and vascular protective activities in the in vitro and in vivo models. The free radical scavenging and antioxidant activities of plant extracts were evaluated in vitro by the 1,1-diphenyl-2-picrylhydrazyl assay, the ferric reducing antioxidant power assay, the intracellular antioxidant activity in rat peritoneal macrophages by dihydrofluorescein assay, and the inhibition of nitric oxide (NO) production in RAW 264.7 macrophages. In an animal model of oxidative stress and vascular dysfunction, male Sprague-Dawley rats were orally administered with aqueous plant extracts (1 g/kg/d) or N-acetylcysteine (NAC; 300 mg/kg/d) as a control for 6 d. On day four, all animals except the normal control group, were administered with phenylhydrazine (PHZ) intraperitoneally. It was demonstrated that the plant extracts possessed high free radical scavenging and antioxidant activities. PHZ induced severe hemolysis and hemodynamic disturbances and treatment with the extracts and NAC significantly improved the hemodynamic status. Vascular responsiveness to bradykinin, acetylcholine, and phenylephrine in PHZ-control rats was markedly impaired, and the plant extracts or NAC largely restored the vascular responses. Moreover, the plant extracts prevented loss of blood reduced glutathione and suppressed formation of plasma malondialdehyde, plasma NO metabolites and blood superoxide anion. It was concluded that the plant extracts possess antioxidants and have potential roles in protection of vascular dysfunction.