Stem bark of Cinchona sp. (Rubiaceae) is one of the well known drugs for its therapeutic values in traditional as well as modern medicine. Even though a lot of work has been carried out on quinoline alkaloids of Cinchona, its phenolic constituents received very little attention. In the present study, we evaluated antioxidant properties of C. officinalis stem bark methanolic extract and water extract containing phenolic compounds (total phenolics 21.37, 5.18% w/w respectively in the two extracts) in different in vitro and ex vivo models viz., antiradical activity by DPPH reduction, superoxide radical scavenging activity in riboflavin/light/NBT system, nitric oxide radical scavenging activity in sodium nitroprusside/Greiss reagent system and inhibition of lipid peroxidation induced by iron-ADP-ascorbate in liver homogenate and haemolysis of erythrocytes induced by phenylhydrazine in erythrocyte membrane stabilization study. Both the extracts exhibited very good antioxidant activity in all the models tested. The phenolic compounds including tannins present in the stem bark seem to offer protection from the oxidative damage.