Abstract: This study was designed to investigated the antioxidant effects and chemical compositions of ethanol extract of Eucalyptus urophylla×Eucalyptus grandis bark. The ethanol extracts of the outer and inner barks were analyzed for potential antioxidant capacity by employing different in vitro assays including reducing power activity,DPPH·scavenge activity and metal chelating activity.A rat aging model induced by D-galactose (120 mg/kg) was used to assess the antioxidant effect of extract from E. urophylla×E. grandis barks,whose serum and tissue index were evaluating indicators including the enzyme activity of superoxide dismutase (SOD),catalase (CAT) and glutathione peroxidase (GSH-Px),as well as malondialdehyde (MDA) level. The outer and inner barks were examined their total flavonoids,tannins and triterpenoids contents by spectrophotometric method. The results showed that,the ethanol extracts from the outer barks and inner barks of E. urophylla×E. grandis exhibited dose-dependent antioxidant activities,the ethanol extracts from E. urophylla×E. grandis outer barks (EEOB) showed higher antioxidant activities than the ethanol extracts from E. urophylla×E. grandis inner barks (EEIB),while the ethanol extracts from EEOB had higher antioxidant activities in reducing power activity and DPPH·scavenging activity than the same concentration of vitamin C. The ethanol extracts from EEOB could increase the enzyme activity of SOD and reduce MDA level in serum,hepatic tissue and kidney of aging mice,meanwhile high concentration (100 mg/L) of extracts from EEOB also significantly increased the CAT and GSH-Px in serum and kidney (p<0.05). The ethanol extracts of EEOB and EEIB were rich in flavonoids,tannins and triterpenoids,with the contents of flavonoids were 6.41% and 2.95%,the contents of tannins were 5.05% and 6.49%,the contents of triterpenes were 11.70% and 4.01%,respectively. Furthermore,based on the results obtained,E. urophylla×E. grandis barks would be considered as a promising source of natural antioxidant compounds.