The present invention discloses use of a colicin Ib and a microorganism expressing the colicin Ib, to improve animal's intestinal microflora, and an effect of improving the growth efficiency and the feed-to-meat rate is obtained. Accordingly, the colicin Ib and the microorganism expressing colicin Ib can be used as a meat-growth agent or as effective gradient for a composition for promoting growth of animals or improving intestinal microflora.
