The objective of this experiment was to investigate the effects of niacinamide on blood in‐ dexes, antioxidant and anti-inflammatory ability of lactating dairy cows, and to provide theoretical refer‐ ence for the application of niacinamide in ruminants. Forty healthy Holstein cows with similar lactation days (170±50) d, parity (2.23±0.62) and milk yield (36.17±7.40) kg/d in middle and late lactation were selected and randomly divided into four groups with 10 cows in each group, they are the control group (CK group) and the experimental groups (group NAM7, NAM11 and NAM15), respectively. Cows in the CK group were fed the basal diet, and group NAM7, NAM11 and NAM15 were given 7, 11 and 15 g/d niacinamide on the basis of CK group, re spectively. The experiment lasted for 75 days, including 15 days of prefeeding-period and 60 days of experimental period. On the last day of the experiment, 20 mL of venous blood was collected from the oxtail root before morning feeding, and serum and plasma were prepared, respectively, for the correlation analysis of blood biochemical indexes, serum antioxidant and inflammatory factors. The results were as follows: the activities of aspartate transaminase, alanine transaminase and alkaline phosphatase in plasma of lactating cows were not significantly affected by administration of different levels of niacinamide (P>0.05); the plasma urea nitrogen content of experimental groups was significantly de‐ creased compared with CK group (P<0.05), and the plasma urea nitrogen content of NAM7 group was lower than that of NAM11 and NAM15 groups, but there was no significant difference (P>0.05); the con‐ tents of total protein and globulin in NAM7, 11 and 15 groups were higher than those in CK group, but there were no significant differences (P>0.05); plasma glucose content of experimental groups was significantly higher than that of CK group (P<0.05); plasma triacylglycerol content in experimental groups was significantly lower than that in CK group (P<0.05), but there was no significant difference among experimental groups (P>0.05). The contents of superoxide dismutase, glutathione peroxidase and catalase in serum of experimental groups were higher than those of CK group, but there were no significant differences (P>0.05); the serum malondialdehyde level in experimental groups was lower than that in CK group, but the difference was not significant (P>0.05). The contents of tumor necrosis factor-α, inter‐ leukin-1β and interleukin-6 in serum of lactating cows were decreased by different levels of niacinamide administration, while the contents of interleukin-10 in serum were increased, but the difference was not significant (P>0.05). It is concluded that niacinamide can reduce the content of urea nitrogen in plasma and improve the utilization rate of nitrogen, and the effect of NAM7 group is better. Therefore, the optimum recommended amount of mesotinamide production is 7 g/d. [ABSTRACT FROM AUTHOR]