Abstract: Eight lactating Holstein dairy cows were divided into two groups (n=4) and used in a double reversal trial with three periods of 14 days each, to evaluate the effects of rumen protected fatty acids and amino acids(Lys and Met) blend(RPLMF) on rumen fermentation, blood metabolism, milk production and composition. The control group cows were fed normal diet, which contained 57% roughage (20% Sudan grass hay, 20% cubed alfalfa hay, 17% Italian rygrass silage ) and 43% concentrate mixture. The experimental group cows were fed 160 g/d RPLMF in addition to normal feed. The RPLMF mainly contains 60%fatty acids, 8% Lys, 4% Met, and 28% others. Rumen fermentation was not affected by the addition of RPLMF. There were no significant differences for the concentration of urea nitrogen, total free amino acid, total essential free amino acid, Lys and Met in blood plasma between the two groups (P＞0.1). The concentration of glucose in blood plasma tended to decrease (P＜0.1), and concentration of His in blood plasma significantly decreased (P＜0.05) with cows fed RPLMF. Dry matter intake was similar between the two groups. Yields of milk and 4% FCM production were 25.6 kg/d and 24.1 kg/d in the experimental group respectively; of that were 24.2 kg/d and 22.3 kg/d in the control group respectively, and compared with the control group, they increased by 1.4 kg/d (5.8%) and 1.8 kg/d (8.1%) respectively (P＜0.1). The percentage of milk fat tended to increase (P＜0.1) for cows fed RPLMF, but percentages of other milk components did not show significant differences between the two groups. Compared with the control group, the quantity of milk fat, increased by 9.5% (80 g/d, P＜0.05), and that of milk protein and SNF increased by 6.9% (50 g/d, P＜0.1) and 6.4% (130 g/d P＜0.1) respectively, and ratio of C18∶0 in milk fat significantly increased (P＜0.05) for cows fed RPLMF. The efficiency of conversion of feed protein to milk protein was increased from 23.9% to 25.3% by the addition of RPLMF. When dairy cows in mid lactation was fed with the RPLMF, its milk production was increased and milk composition was improved in a hot season.