Abstract: Organic acids, as growth promoter, have antimicrobial effects on animals, through different mechanisms. In this review, five possible mechanisms of organic acids are discussed including: energy competition permeabilizing through outer membrane, increase of cellular osmolarity, inhibition of macromolecule synthesis and induction of antimicrobial peptide in host cell, at levels of molecular mechanism, respectively. This review concludes that, the intracellular accumulation of anions of organic acid is a primary contributor to antimicrobial effects, which can be influenced by differences or shifts between intracellular and extracellular concentration of pH and anion concentration of extracellular acids.