The effect of water extract from Taraxacum mongolieum Hand-Mass (TMW) on proliferation of human breast cancer cells, nitric oxide production and nitric oxide synthase expression was tested. TMW inhibited the growth of both estrogen-dependent MCF-7 and estrogen-independent MDA-MB-435 human breast cancer cells. Lipopolysaccharide-induced nitric oxide (NO) production was reduced by TMW in a concentration-dependent manner. Expression of inducible nitric oxide synthase (iNOS) was also suppressed with the treatment of TMW in Raw 264.7 cells. Therefore, Taraxacum mongolieum Hand-Mass is worth further investigation with respect to breast cancer therapy.