To examine the action mechanism that mediates the anti-fertility effect of Costus speciosus extract, research was conducted on male Sprague-Dawley rats. Costus extract was given to male rats for 14 days at various doses, namely 275, 550 and 1,100 mg kg-1 day-1 in 0.5% sodium CMC. The results showed that Costus extract with doses ranging from 275 to 1,100 mg kg-1 day-1 was able to inhibit pregnancy among female rats by 10-70%. No obstacles in terms of sexual behavior were identified among male rats. The anti-fertility effect of Costus extract kicked in without involving a decreased level of male reproductive hormones.