Abstract: The experiment was designed by using the randomized block trial ways. 48 Holstein cows with similar parity, milk yield and lactation period were selected and divided into four groups in the same basal diet.The treatment groups were used to evaluate the effect of addition of rumen-bypass amino acid on the milk production,and milk components.Four treatments are as follows： the control group no rumen-protected lysine（RPLys） supplementation; treatment one （addition 10 g RPLys daily for each cow ）;treatment two （addition 20 g RPLys daily for each cow）; treatment three （addition 30 g RPLys daily for each cow）.The results showed that compare with the control group,the milk yield of the experimental groups increased significantly. The milk yield of treatment three （P0.01） and treatment two （P0.05）was significantly higher than the control group.The milk protein percentage was tended to increase, and the treatment three was significantly higher than the control group（P0.05）.Other milk components content also increased slightly,but no significant difference was found （P 0.05）.