Twenty four multiparous Holstein cows in their early stage of lactation with a mean of 59±20 DIM and a mean of 615±40.5 Kg of BW were employed in a complete randomized block design to evaluate the effects of different levels of sugar beet pulp in a partial replacement for alfalfa, the only source of forage in their previous diet (treat 1, 0% beet pulp and 35% alfalfa, treat 2 ,6% beet pulp and 28% alfalfa ,and treat 3 ,12%beet pulp and 22% alfalfa) on milk yield and composition, dry matter intake, chewing activity , rumen and fecal pH, blood metabolites and digestibility of nutrients. The experiments were based on the new effective fiber hypothesis. Diets were formulated to contain 19.3% crude protein and 32.5% neutral detergent fiber.Cows were divided in three equal groups, each group receiving one of the diets in the form of TMR. There was not any treatment effect observed on DMI, milk yield and composition, and on fat corrected milk. Time of ruminating activities were significantly different among treatments (p<0.05).Chewing activity per kilogram of DMI, and forage NDF intake also were of significant differences(p<0.05). Rumen pH was of significant difference among treatments (p<0.05). Experimental diets did not show any significant effects on fecal pH, BW and BCS during the whole experimental period. Blood metabolites and digestibility of nutrients did not also differ among treatments.