Three Luxi yllow cattle (350±9 kg) were used in 3×3 Latin Square Design to evaluate the influence of varying ruminally degradable protein (RDP) to ruminally undergradable protein (RUP)ratio on ruminal fermentation and digestibility of distilled grain, chinese wildrye and corn silage.Three experiment diets A, B and C were formulated to contain RDP∶ RUP of 50∶50, 55∶45 and 60∶40,respectively.The cattles were fed ad libitum.Ruminal pH and NH3-N concentration was not affected with the different RDP to RUP ratio.MCP,TVFA and propionate concentration by cattles fed C diet was higher (P<0.05) than those fed A and B.NDF digestibility was increased (P<0.05) as the proportion of RDP increased in all three roughages.Crude protein digestibility remained unaltered across all treatments.Our data suggest that the rumen microbial growth can be increased by increasing RDP to RUP ratio and diet containing RDP to RUP ratio 60∶40 (1.5) is optimum for ruminal fermentation and NDF digestibility.