5-Alpha reductase Ⅱ, which converts testosterone (T) to dihydrotestosterone (DHT), is a crucial enzyme Ⅱ in benign prostatic hyperplasia. Inhibitory effects of Curcuma longa L. (CL) extracts on 5-alpha reductase Ⅱ activity were investigated in rat prostate tissue homogenates as well as LNCaP cells expressing human 5-alpha reductase Ⅱ. Hot water extract (CL-HW) of Curcuma longa L. significantly inhibited 5-alpha reductase activity by over 80% at a concentration of 100 μg/mL, whereas 20% ethanol extracts (CL-E20) of Curcuma longa L. exhibited significant inhibitory activity from 50 μg/mL. These results indicated that Curcuma longa L is a potent 5-alpha reductase Ⅱ inhibitor for benign prostatic hyperplasia treatment.