Prostate cancer is the second leading cause of male deaths due to cancer in the United States. Hydrogenated vegetable oils have been suspected of inducing adverse health effects, including atherosclerosis and cancer. Here we report that a selectively hydrogenated soybean oil (SHSO) containing a high quantity of conjugated linoleic acids showed remarkably strong anticarcinogenic activity against prostate cancer in the rat model (Copenhagen rats with MAT-LyLu syngeneic rat prostate cancer cells) study in vivo and human prostate carcinoma cell lines studies in vitro, as compared with native soybean oil. A 5% dietary supplementation with SHSO inhibited the growth of prostate cancer by 80% in vivo. The TUNEL method and immunohistochemical staining assays of bax, bcl-2, and survivin clearly showed that SHSO induced prostate cancer cell apoptosis in the tested rats. DNA fragmentation analysis in vitro further confirmed the apoptotic activity of SHSO on the MAT-LyLu prostate cancer cells. The SHSO also showed strong cytotoxicity on human prostate cancer cells (DU145 and PC3). This represents the first report demonstrating the significant anticancer activities of hydrogenated vegetable oils at low levels of dietary supplementation.