Benign prostate hyperplasia (BPH) is the most common proliferative disorder affecting older men and results in prostate enlargement and lowered urinary tract symptoms. Jaeumganghwa-tang (JGT), an oriental traditional herbal formula, has been used in China (Zi-yin-jiang-huo-tang in Chinese), Japan (Jiin-koka-to in Japanese), and Korea for many years. Effects of JGT on prostate dihydrotestosterone (DHT) level and prostatic hyperplasia were investigated using a rat model, in which BPH was induced using testosterone propionate (TP). Rats were divided into five groups. One group was used as a normal, and four groups received subcutaneous injections of TP for 4 weeks to induce BPH. JGT (200 or 400 mg/kg) was administered daily for 4 weeks by oral gavage concurrently with TP injections, and rats were sacrificed at scheduled times. Prostates were weighed, and histopathologic examination was conducted. DHT levels in serum and the prostate were measured, and the expression of proliferating cell nuclear antigen (PCNA) protein was investigated using Western blotting. BPH animals showed increases in absolute and relative weights of the prostate, levels of DHT in serum and the prostate, and expression of PCNA in the prostate, whereas JGT-treated animals showed significant reductions in these indices compared with the BPH animals. Administration of JGT attenuated TP-induced epithelial hyperplasia. These findings indicate that JGT inhibits the development of BPH, an effect closely associated with a reduction in DHT level.