The methanol extract of the aerial part of Tessaria integrifolia (common name: pajaro bobo), which is used as an herb for the treatment of asthma and liver injury in Peru, showed a stronger antioxidative activity than the standard synthetic antioxidant, 3-tert-butyl-4-hydroxyanisole (BHA). From this extract, five new eudesmane-type sesquiterpenoids were isolated along with eighteen known compounds, and their structures were elucidated on the basis of MS, ¹H-NMR and ¹³C-NMR data. Among them, eleven phenolic compounds, four flavonoids, two lignans and five caffeoyl quinic acid derivatives were examined for their antioxidative activity using the ferric thiocyanate method, and nine compounds indicated a stronger antioxidative activity than the standard natural antioxidant, α-tocopherol at a concentration of 0.5 mM. Especially, two flavonoids were identified as stronger antioxidants than BHA. The scavenging effect of the phenolic compounds on the stable radical 1,1-diphenyl-2-picrylhydrazyl was examined. All tested compounds showed this scavenging effect, with the effect of five caffeoyl quinic acid derivatives and three flavonoids being almost twice or more that of α-tocopherol at a concentration of 0.01 mM.