The antioxidant activities against fish oil oxidation of six commercially available flavonoids and of five flavonoids purified from two Chilean native plants were compared to those ofdl‐α‐tocopherol and of two synthetic antioxidants, butylated hydroxytoluene and butylated hydroxyanisole. Among the commercial flavonoids, catechin, morin and quercetin showed a higher activity when fish oil oxidation (either spontaneous or Fe2+‐induced) was assessed from the formation of peroxides or thiobarbituric acid‐reactive substances. Among the native flavonoids, the 5,3′,4′‐trihydroxy‐7‐methoxy flavanone (designated as Pt‐2) showed the highest antioxidant activity. Mixtures of quercetin or of Pt‐2 withdl‐α‐tocopherol produced better inhibitory effects when compared to that of each substance assayed by itself. Also, when Pt‐2 and quercetin were assayed in combination (0.3 g/kg oil and 0.7 g/kg oil, respectively), a synergistic antioxidant effect was observed. Results indicate that several flavonoids could be used as natural antioxidants as a means to replace those synthetic antioxidants, the use of which has been questioned.