The cytotoxicity, scavenging and lipid peroxidation - inhibiting activities of 5,3 ',4'- trihydroxy - 7 - methoxyflavanone, isolated from Haplopappus rigidus Phil.(Asteraceae), were evaluated in a series of in vitro assays involving free radicals (ABTS, DPPH), ascorbate - induced rat liver microsomal lipid peroxidation, and the cytotoxicities to three human tumor cell lines . 5,3',4'-trihydroxy - 7 - methoxyflavanone , at concentration of 0.3 mg/mL , elicited an 97.4 % scavenging effect on DPPH; TEAC 1.35mM on ABTS , preventing microsomal lipid peroxidation IC50 1.8 μM and ED50 3.23 - 5.66 ug/mL against three human tumor cell, indicating that the flavanone 5,3',4'-trihydroxy - 7 - methoxyflavanone was effectively cytotoxic , as well as free radicals scavenger and a good inhibitor of iron/ascorbate induced lipid peroxidation in a dose dependent manner.