The objective of this study was to evaluate the effects of diet supplementation with live yeast (Saccharomyces cerevisiae) and mannan-oligosaccharides (MOS) on ruminal parameters， nutrient digestibility， and the inflammatory response in cattle fed grain-based diets. Three Holstein steers (body weight of 497±3 kg) with ruminal and duodenal cannulas were assigned to a 3 × 3 Latin square design. The animals were kept in individual pens and fed a diet containing 5% sugarcane bagasse and 95% concentrate (906.5 g/kg ground corn). Diet treatments were Control (without additive)， Yeast (1.5 g/kg DM live yeast Saccharomyces cerevisiae， NCYC 996) and MOS (1.5 g/kg DM MOS， β-glucans and mannan). Dry matter intake， ruminal， intestinal， and total digestibility of nutrients were not affected by the treatments. The ruminal concentration of isobutyric acid increased in animals fed on diets supplemented with Yeast and MOS， whereas isovaleric acid increased with Yeast and decreased with MOS supplementation. Dietary supplementation with Yeast and MOS increased pH and decreased ammonia concentration in the rumen. Lipopolysaccharide (LPS) concentrations in the rumen and duodenal fluid were not influenced by the additives. However， both Yeast and MOS decreased the plasma levels of LPS and serum amyloid A (SAA). In conclusion， when high-concentrate diets fed to beef cattle are supplemented with either Yeast or MOS， ruminal pH is increased， LPS translocation into the blood stream is decreased， and blood SAA concentration is decreased. These factors reduce the inflammation caused by consumption of grain-based diets， and either supplement could be used to improve the rumen environment in beef cattle susceptible to ruminal subacute acidosis.