Breast cancer is one of the most lethal diseases world-wide. However, there is a large difference in breast cancer incidence among Caucasian, Hispanic, African and Asian (e.g. Chinese) women with Caucasian women being the highest and Asian women being the lowest. It has been suggested that the dietary factors may account for approximately 50% of the breast cancer. One of such dietary components which are typical to Asian but not Caucasian diet is soy foods. A number of epidemiological studies have suggested that increasing soy consumption could be related to the decreased risk of occurrence and/or mortality of breast cancer. In this review, we first described briefly different types of soy products and their nutritional functions and consumption. Then, we described briefly soybean isoflavones, i.e. genistein (GEN), daidzein, glycitein, and presented several lines of evidence to demonstrate the possible association of soy flavone food consumption with incidence and prognosis of breast cancer; finally, we summarized several possible molecular mechanisms, including the effects of GEN as an agonist of ERβ, epigenetic and genome-wide effects, activation of peroxisome proliferator-activated receptors, induction of apoptosis and stimulation of autophagy, involved in the chemo-preventive effects of GEN on breast cancer.