Abstract: Twenty-one dairy cows with lumen cannulas were divided into seven groups randomly which were ted on TMR （control） and TMR added 50.0,100.0,150.0,200.0,250.0,300.0 g/（head·day） urea （ Ⅰ -Ⅵ treatments） respectively. The concentrations of BUN,MUN ,RUN and lactation performance of dairy cows were determined in this trial. The results were as follows： compared with the control, the concentrations of BUN and MUN of group Ⅰ -Ⅲhad no difference and the milk production and composition increased; the concentrations of BUN and MUN of group Ⅳ-Ⅵ were significantly higher than that of control, however,the milk production and fat percentage decreased and the percentage of milk protein and concentration of milk sugar decreased significantly.There was high relationship between the concentrations of B U N and M UN of dairy cows（r=0.8974,P〈0.01）.