The present disclosure relates to a feed additive composition for reducing methane production generated in the ruminant stomach of ruminant animals. Specifically, the present disclosure relates to a feed additive composition for reducing methane production comprising at least one selected from the group consisting of alliin and berberine, more specifically to a feed additive composition for reducing methane production comprising at least one selected from the group consisting of diallyl disulfide (DADS), nitrate, and eucalyptus oil. The feed additive composition according to the present disclosure may be added to the feed without additional treatment, thereby reducing the amount of methane production in the ruminant stomach without negative effects on livestock productivity. Thus, the feed additive composition can be effectively used in the ruminant animal industry.