Poncirus trifoliata was evaluated for antioxidant potential by several radical-scavenging assays, including 1,1-diphenyl-2-picrylhydrazyl, superoxide, hydroxyl, and alkyl radicals, using electron spin resonance technique. P. trifoliata extracts were prepared by using water and methanol, partitioning with different organic solvents. Among the tested samples, water extract and dichloromethane fraction evidenced relatively higher scavenging activities than those of other samples on reactive radicals and have high amount of antioxidant component such as flavonoids ($8.5{pm}0.3$ and $8.3{pm}0.2mg$ rutin equivalents/g dry weight basis) and proanthocyanidin ($2.6{pm}0.3$ and $2.8{pm}0.5mg$ catechin equivalents/g dry weight basis), respectively. Moreover, the water extract showed significantly high enhancement of cell viability against hydrogen peroxide-induced oxidative damage as well as reduction in the hydrogen peroxide-induced intracellular reactive oxygen species on CCL 13 cell line. These results indicate that P. trifoliate exerts various antioxidant effects against several reactive radicals and hydrogen peroxide-mediated cell damage. Thus, P. trifoliate could be a potential therapeutic modality for the treatment or prevention of several diseases associated with oxidative stress.