Abstract: The experiment was performed according to a 3 ×3 Latin square design.Nine sheep implanted with rumen cannulas were selected.They were fed with diets based on guinea grasses,alfalfa hay and concentrates （concentrate：hay,4：6） supplemented corn oil at a concentration of 0,4% and 8%,respectively.Rumen fluid samples were collected on 0,2,4,6,8,10 and 12 h post morning feeding.The pH value,NH3-N concentration,VFA concentration and BCP concentration were determined.The results showed that supplementation of corn oil at a concentration of 4% or 8% increased the rumen pH value and decreased the VFA concentration,acetic acid concentration and ratio of acetic acid to propionic acid,but it had no effects on the NH3-N concentration and propionic acid concentration.Supplementation of corn oil at a concentration of 4% decreased the BCP concentration,and supplementation at a concentration of 8% decreased butyric acid concentration.