The subject of this study that was conducted in three separate experiments. In the first experiment, 12 lactating Holstein cows (2 parities, 50 ± 10 days in milk and the average milk production of 45 kg) used 4 × 4 Latin square design that included 4 treatments and 3 replications (cows). In the second experiment, 9 lactating Holstein cows (2 parities, 50 ± 12 days in milk and the average milk production of 48 kg) used 3 × 3 Latin square design that included 3 treatments and 3 replications (cows). Treatments 1 to 4 in the first experiment consisted of diet containing 18, 17.2, 16.4, and 15.6 percent crude protein, respectively with the 18 percent crude protein diet considered the control group. Rumen degradable protein levels were constant across the treatments (approximately 10.9 percent DM basis) while rumen undegradable protein was gradually decreased. Treatments 1 to 3 in the second experiment consisted of diet containing 16.4, 15.6, and 14.8 percent crude protein, respectively with the 16.4 percent crude protein diet considered the control group. rumen undegradable protein levels were constant across the treatments (approximately 5.6 percent DM basis) while rumen degradable protein was gradually decreased. In the three experiment consist of 18, 16.4 and 14.8 % CP that use select treatment from 1 and 2 experiments on 300 head dairy herd n reproductionperformance. In the first experiment the diets had no significant effect on milk production and composition. But dry matter intake and 4 percent fat-corrected milk in treatment 3 was significantly higher than the other groups. Apparent digestibility of dietary protein in the first treatment was more than other groups. In the second experiment milk urea nitrogen due to decrease rumen degradable protein significantly decreased, and a numerical decrease in rumen ammonia-N concentration was observed. Results of the three experiments showed that the decrease in the level of crude protein, rumen undegradable protei and rumen degradable protein (respectability, 14.8, 4.7 and 9.3 percent DM) have no negative effect on milk production and composition, DMI and reproduction performance.