The objective of this study was to replace the wheat straw with different levels of sugar cane bagasse and study its effect on ruminal total fermentation, chewing activity, lactation performance, nutritional intake and digestibility,.dietary.intake.and.milk.yield.in.mid.lactation.Holstein.cows.Inthis.experiment,.9.multiparous.Holstein.cowsaveraging.105.±.12.1.d.in.milk.42.1 ± 2.94 kg of milk/d, body weight 617 ± 59.5 and body condition score 2.94 ± 0.3 (mean ± SD) were used in a 3 x 3 Latin square three 21-d experimental periods. Experimental treatments included: 1) control diet containing 100% wheat straw, 2) replacement of wheat straw with sugar cane bagasse at 33 percent level, and 3) replacement of wheat straw with cane bagasse at 67 percent level. Diets were balanced on the basis of energy and protein. The replacement of wheat straw with sugar cane bagasse resulted in a linear increase (P 0.01) in residual particles on a 19 millimeter sieve and a linear decrease (P 0.01) in residual particles on a sieve of 8 millimeters; the remaining particles on the sieve of 1.18 millimeters increased (P = 0.01) by 67 percent in sugar beet bagasse, but Did not make any difference in the remaining particles on the Peylvania Tray. Effective physical fibers larger than 1.18 mm were also linearly increased (P = 0.01). The replacement of wheat straw with sugar cane bagasse on the pattern of dietary behavior did not affect the total time of eating, the replacement of wheat straw with sugar cane bagasse on the pattern of dietary behavior did not affect the total time of eating, chewing time and total chewing time increased linearly in sugarcane bagasse containing treatments than control treatment, but the resting behavior pattern was linearly decreased in treatments containing sugarcane bagasse. Milk production, corrected milk for 3.5% fat and milk compositions were not affected by experimental treatments. The use of uNDF 288 as a percentage of NDF and dry matter increased linearly (P 0.01) with the increase in the sugar cane bagasse level of the diet and pdNDF in the treatment of 67% bagasse of sugar cane was reduced linearly (P 0.01) compared to the other two treatments. The replacement of wheat straw with sugar cane bagasse did not have any effect on the consumption of dry matter, fat and NDF, and resulted in the same efficiency of the feed, but, organic matter consumption decreased by 67% treatment in comparison to the other two treatments linearly (P 0.01) and protein intake was reduced linearly in sugar cane bagasse treatments (P = 0.01). The apparent digestibility of dry matter, organic matter, NDF and protein decreased linearly with increasing sugar cane bagasse content in the diet (P 0.01), Ruminal pH was affected by experimental treatments, so that in treatment of 67% sugar cane bagasse decreased than the other two treatments but the pH of fecal and urine was not affected by experimental treatments. nitrogen intake, milk nitrogen, urine nitrogen and ammonia nitrogen were not affected by experimental treatments, However, fecal nitrogen was affected by experimental treatments and 67% of sugar cane bagasse treatment decreased linearly than the other two treatments (P = 0.02). By increasing the level of sugar cane bagasse in the diet, BUN concentration increased linearly in treatments containing sugar cane bagasse (P = 0.02). The replacement of wheat straw with cane bagasse reduced the price per kilogram of dietary dry matter and resulted a numerically increase in the sale of milk. The results of this experiment showed that the replacement of wheat straw with different levels of sugar cane bagasse in the Holstein dairy ration in the middle of lactation did not affect their yield, and sugar cane bagasse could be an appropriate alternative to wheat straw. key words: Wheat straw, Sugar cane bagasse, uNDF, lactating dairy cow performance