The objective of his study was to investigate the effect of fructooligosaccharides (FOS)infusion in the rumen on rumen fermentation.Six wethers fitted with permanent ruminal cannulas were randomly allocated to two groups that were infused with FOS at 1.00% and 2.00% of daily feed intake, respectively.Significant (p<0.05) decreases in pH value and NH3-N concentration in the rumen liquid were observed after continuous FOS infusion at both levels of 1.00% and 2.00% of daily feed intake.Meanwhile bacterial protein (BCP) production and volatile fatty acids (VFA)concentration in the rumen liquid were both significantly enhanced (p<0.05).There was a higher decrease (p<0.05) of NH3-N concentration in the rumen infused with 2.00% FOS than those infused with 1.00% FOS.However, no significant difference (p>0.05) was observed in ruminal pH value,VFA concentration and BCP production between two infusion levels of FOS.Results from this study indicate that FOS could improve the rumen fermentation of growing sheep.Further studies are warranted to determine the optimal addition level in the diets.