We reported previously that the extracts of Yacon tuber and leaf have spermatogenic activity and testosterone level increasing effect in men. The aim of this study was to investigate the mechanism of testosterone level increasing effect of Yacon tuber and leaf extracts. In this study, we found that Yacon leaf extracts and chlorogenic acid, a constituent of Yacon extracts showed significantly inhibition effect of testosterone degradation in rat liver homogenate. We suggest that this could be a part of direct mechanism about increasing effect of testosterone level. Futhermore, we evaluated the inhibition effect of testosterone metabolism of Torilis japonica and Coix lacryma-jobi have been reported as $5{alpha}$-reductase inhibitor. The inhibition ratio of testosterone metabolism of these were Torilis japonica>Coix lacryma-jobi>Yacon. We also evaluated the benign prostatic hyperplasia (BPH) score with the preparation composed of these natural product, Yacon pill. It showed the decreased the BPH score in men. Rats with BPH showed significantly increased dihydrotestosterone (DHT) levels in the serum; however, Yacon pill extracts treated rats showed significant reduction of DHT levels compared with the BPH group. Conclusively, Yacon pill showed the possibility as ameliorable agents of BPH symptoms.