The experiment was conducted to study the effects of dietary cation-anion balance (DCAB) on blood physiological indices and performance in the dairy cows in mid-lactation. Twelve dairy cows in mid-lactation period were used in the experiment and divided into 4 groups randomly with 3 cows in each group to determine the optimum level of DCAB. The experiment lasted for 21 days, and four levels of DCAB were as follows: group A (DCAB of 200 mEq/kg DM), group B (DCAB of 250 mEq/kg DM), group C (DCAB of 300 mEq/kg DM), and group D (DCAB of 400 mEq/kg DM). The results showed as follows: abilities of blood buffer and O2 transport of dairy cows were increased with DCAB increasing. Blood pH and content of HCO-3 were increased predominantly as a result of DCAB increase (P<0.05). The partial pressure of CO2 in arterial blood in group B had significant difference in comparison with that in groups C and D (P<0.05). Group C had significant influence on TCO2 and blood base excess in comparison with group A (P<0.01). The increase of DCAB affected lactation performance, but not significantly (P>0.05). Considering of the results above, a better effect of diary cows was observed in group C with DCAB of 300 mEq/kg DM.［Chinese Journal of Animal Nutrition, 2010，22（4）:1096-1100］