Lithocarpus polystachyus leaves have been used as tea beverage and folk medicine for healthy care in the Southwest of China. The purpose of this study is to investigate the anticancer activity of Lithocarpus polystachyus Rehd leaf aqueous extract (LPAE) and to explore the possible mechanism of its activity. Growth inhibition effects of LPAE breast cancer were tested in vitro and in vivo. The possible mechanism of its activity was analyzed with cell biological and molecular biological assays. After LPAE treatment, the proliferation and colony formation of cancer cells decreased; apoptotic cells increased; DNA fragmentations were evident; mRNA and protein expressions of PPARγ, Bax, and caspase-3 genes increased and expressions of cyclin D1 and Bcl-2 genes decreased; in vivo experiment, LPAE inhibited human beast cancer growth. The findings in this experimental study suggested that LPAE has potential cytotoxic and apoptotic effects on human breast cancer cells in vitro and inhibits the cancer growth in vivo, and its mechanism of activity might be associated with apoptosis induction of cancer cells through upregulation of the mRNA and protein expressions of PPARγ, Bax, and capase-3 genes and downregulation of the expressions of cyclin D1 and Bcl-2 genes.