Marine-derived n-3 polyunsaturated fatty acids (PUFA) are shown to inhibit mammary carcinogenesis. However, evidence regarding α-linolenic acid (ALA), a plant-based and major n-3 PUFA in Western diet, remains equivocal. This study examined the inhibitory potency of plant- versus marine-derived n-3 on mammary tumorigenesis. Female MMTV-neu(ndl)YD5 mice were lifelong exposed to one of four oil diets: 1) 10% safflower (n-6, control), 2) 10% flaxseed, or 7% safflower plus either 3) 3% flaxseed, or 4) 3% menhaden. Compare to control, 10% flaxseed and 3% menhaden oil had similar inhibitory effects, which significantly reduced terminal end buds, and decreased overall tumor outcomes by regulating tumor protein expression (p<0.05). A significant dose-dependent reduction on tumor outcomes were observed in mice fed 3% and 10% flaxseed oil (p<0.05). However, 3% flaxseed had weaker inhibitory potency compared to 3% menhaden oil (p<0.05). Overall, marine-based n-3 PUFA are more potent than plant-derived ALA in mitigating mammary tumorigenesis.