Angelica gigas Nakai (AG) has been studied for various pharmacological actions in traditional oriental medicine. In this study, we investigated the effects of AG on benign prostatic hyperplasia (BPH) models induced by castration and testosterone propionate (TP) injection. Thirty rats were divided into six groups. One group was used as a normal control and the other groups were castrated and injected intraperitoneally with TP to induce BPH. A positive control group received finasteride (3 mg/kg). Groups administered AG (1.25, 5, or 10 mg/kg) instead of finasteride were assigned as experimental groups. The prostate volume of the BPH + AG (AG10; 10 mg/kg of bw/day) group decreased by approximately 35% compared to that of the BPH induced (NCON) group. The prostate weight ratio of the BPH + finasteride group decreased by 10% compared to that of the NCON group, while it decreased by 24% in the AG5 group and 22% in the AG10 group. The AG10 group showed the lowest levels of 5α-reductase and dihydrotestosterone. In the histopathological observation of prostate tissue, the control and AG groups showed normal cell shapes and lower intraluminal polyp formation. Therefore, administering 10 mg/kg of AG is considered optimal for the protective effect against BPH.