Abstract: 20 Chinese Holstein cows of basically the same condition at age and lactation were selected, randomly divided into 2 groups of 10 cows. This experiment was assigned to 2 dietary treatments: control group (0%) and NCG group (20 g/d) to study the effects of N-carbonyl glutamic acid added into dairy diet on yield of dairy cattle. The results indicated that the milk production and milk protein rate of the NCG group was higher than that of the control group; the fat in every milk samples was lower. The economic return of the NCG group was higher than that of the control group. It could be concluded that NCG could indirectly affect milk yield and components of milk by urea cycle.