Our objective was to investigate effects of probiotics (P1 DCP) in mid lactation high producing (45.35± 5.68 kg/d) dairy cows. The study was conducted with 56 lactating multiparous Holstein cows (96.87 ± 32.15 d in milk) in a randomized complete block design with two treatments. 1) control: Total mixed ration: 22.5% alfalfa hay, 12.7% corn silage, and 64.8% concentrate on a dry matter (DM) basis (with 300 mg/d/cow of monensin), 2) total mixed ration with different doses of probiotics for 3 periods of time: first period ( rumen Stability period), from day 1 to 10 each cow consumed 56 g P-One/day (each gram of P-One contains 8.84×10 7 cfu lactate producing bacteria, enzymes (fungal and bacterial amylase, fungal protease, fungal beta-glucanase and hemicellulase), 6.6×10 7 cfu yeast, yeast culture and Mannan Oligosaccharide), second period ( seeding period ) from day 11 to 40 each cow consumed 28 g P-One and 28 g DCP/day (each gram of DCP contains 3.5×10 7 cfu propionibacteria, above enzymes and 6.6×10 7 cfu yeast, and third period ( maintenance period) from day 41 to the end of experiment each cow consumed 28 g P-One/day. In the third period diet formulation changed. Each group of cows was in the separate pen. Milk production and samples for milk components, somatic cell counts, digestibility of diet and rumen fluid collected randomly every week. Monensin (control) numerically increased milk production (46.08 vs 44.67 for control and probiotics respectively) and decreased milk fat percentage (3 vs 3.33 for control and probiotics respectively) (P?0.04). There were no effects of treatments on other milk components and somatic cell counts. These additives did not affect neutral detergent fiber, acid detergent fiber and crude protein digestibility. Rumen pH and total volatile fatty acids concentrations measured at 3 h after feeding, were unaffected by treatments. Feeding probiotics numerically decreased molar proportions of acetate but increased molar proportions of propionate by 9.8% (P 0.01), decreased molar proportions of butyrate (P 0.01) and acetate: propionate ratio (P 0.05) compared with control cows. Additionally, with changing the diet formulation in the third period, feed costs decreased by 7.78% /cow/d (? 8518 Rials) in probiotics fed cows. Under the conditions of these studies, supplementing midlactation high producing cows with probiotics, did not affect cow performance and diet digestibility but improved rumen fermentation and adaptation for high concentration diet. We concluded that probiotic protocols might have potential as an effective direct-fed microorganism to improve rumen fermentation in dairy cows. Key words: probiotic, monensin, dairy cow, mid lactation.