This study was conducted to investigate the effect of ruminal infusion of acetic acid on the ruminal pH, volatile fatty acids (VFA) and protozoa in the sheep. Two fistulated sheep were infused through rumen fistula with 500 ml water (control) or acetic acid (0.62 gr/kg body weight) before feeding for 4 consecutive days. Samples of rumen contents were collected from each sheep before and 1 h after, infusion through the ruminal fistula. Acetic acid infusion has caused a significant drop in the ruminal pH. Furthermore, there was a reducing effect of acetic acid on number of ruminal protozoa. Especially, the Holotrich protozoa were the most susceptible, Whereas Entodinia were the least sensitive to ihe reducing effect of acetic acid. While the proportions of Holotrich species decreased, the proportions of Entodinium species were generally found to be increase after acetic acid supplementation into the rumen of sheep. Compared with the values of same parameters in the sheep given water, acetic acid proportions and total VFA levels were determined to be higher, but the proportions of propionic and butyric acids were found lower levels in the sheep given acetic acid.