The inhibitoryeffects of a water extract of sweetpotatoleaves (WSPL) on nitricoxide (NO) production and protein tyrosine residue nitration were investigated. The results showed that WSPL inhibited NO production in a concentration-dependent manner. In the range of 0–1.0 mg/ml, the inhibitoryeffect on NO generation in macrophages increased with increasing concentration of WSPL. Meanwhile, the protein tyrosine residue nitration in mouse heart homogenates was inhibited by 1 mg/ml WSPL. In addition, WSPL, in the range of 0–0.4 mg/ml, also exhibited radical scavenging, reducing and chelating activities and protected liposomes against oxidative damage. A high performance liquid chromatography analysis revealed that phenolic acids and flavonols such as chlorogenic acid, caffeic acid, and quercetin were present in the WSPL, which could contribute to the protective effect against oxidative damage. Thus, WSPL might be useful in preventing proteinnitration and oxidative stress.