Polyunsaturated fatty acids (PUFAs) possess anti-cancer action both in vitro and in vivo.In the present study,we detected cell viability with methyl thiazolyl tetrazolium (MTT) assay and cell membrane permeability with propidium iodide (PI) fluorescence dyeing,and calculated cell membrane fluidity change as fluorescence anisotropy.Fatty acid content in cells was measured by gas chromatography/mass spectroscopy (GC/MS),and the relationship between fatty acid composition and cell viability was studied.We observed that n-6 PUFA linoleic acid (LA) inhibited tumor cell growth at high concentrations (≥300 μmol/L),while low concentrations (100–200 μmol/L) seemed to promote cell proliferation.Analyses of cell membrane permeability,cell membrane fluidity,and cell fatty acid composition suggested that the anti-cancer action of LA could be related to changes in the ratio of n-6 to n-3 PUFAs.We observed that pre-incubation of cancer cells with 100 μmol/L LA for 24 h enhanced cell sensitivity to the cytotoxic action of LA,whereas undifferentiated cell line LoVo seemed to have a distinct path in LA-induced death.These results showed that one of the mechanisms by which supplementation of LA induces cancer cell death could be altering the ratio of n-6/n-3 PUFAs,and this may be related to cell differentiation status. The PUFAs possess anti-cancer action both in vitro and in vivo. The present study, we detected cell viability with methyl thiazolyl tetrazolium (MTT) assay and cell membrane permeability with propidium iodide (PI) fluorescence dyeing, and calculated cell membrane fluidity change as fluorescence anisotropy. Fatty acid content in cells was measured by gas chromatography / mass spectroscopy (GC / MS), and the relationship between fatty acid composition and cell viability was studied. We observed that n-6 PUFA linoleic acid ( LA) demonstrated tumor cell growth at high concentrations (≥300 μmol / L), while low concentrations (100-200 μmol / L) seemed to promote cell proliferation. An assay of cell membrane permeability, cell membrane fluidity, and cell fatty acid composition suggested that the anti-cancer action of LA could be related to changes in the ratio of n-6 to n-3 PUFAs. We observed that pre-incubation of cancer cells with 100 μmol / L LA for 24 h enhanced cell sensitivit y to the cytotoxic action of LA, and undifferentiated cell line LoVo seemed to have a distinct path in LA-induced death. These results showed that one of the mechanisms by which supplementation of LA induces cancer cell death could be altering the ratio of n- 6 / n-3 PUFAs, and this may be related to cell differentiation status.