This study was carried out to determine whether the addition of rumen protected Met to ration formulation allowed a reduction in dietary crude protein (CP) without jeopardizing total milk or milk protein yields. Twenty-eight multiparous Holstein-Friesian cows, in a commercial dairy herd, were randomly assigned to the trial. Rations differed in percentages of CP level (H, 19.3% or L, 16.9%), with or without supplementation by protected methionine. Milk yield was recorded individually at each milking and milk samples were taken during the experimental period. In addition, four digestion trials were conducted on three cows of each group to evaluate the digestibility coefficients of used diets using Acid Insoluble Ash technique. No effects of pMet supplementation or interactions between CP levels and methionine supplementation were detected on any of the parameters measured for digestibility coefficients of nutrient. In early lactation stage, milk production response to pMet supplementation was greater with the low level of CP diets compared with the high level (2.00 vs. 0.300 kg/d, respectively). The pMet supplementation by CP level interaction was significant for protein concentration, while it was not significant for fat and lactose concentration during early and mid lactation period. Every one percentage unit increase in dietary CP above 16.9%, MUN concentrations were, in average, increased 2.2, 2.0, and 1.6 mg/dl in early, mid, and late lactation, respectively. The feed and N utilization efficiencies were improved by feeding low CP diets supplemented with pMet in early lactation. Cows fed diets reduced in CP level had a reduction in cost of feed per kg milk in various stages of lactation. In early lactation stage, feeding diets supplemented with pMet reduced the cost of feed per kg milk by 0.04 L.E. It could be concluded that, a reduction in dietary protein to 16.9% and supplemented with protected AA, particularly the first limiting AA, is an acceptable new nutritional strategy for improving productive performance in dairy cattle and may be used as an approach for reformulating lactating cows rations based on AA profile rather than CP as simple or metabolizable protein. This strategy, with prevailing feed prices, could be compatible with maximum profit and a moderate amount of nitrogen excretion to the environment.