Abstract: An experiment was conducted using three Chinese Holstein cows in early lactation under hot environment,in a 3×3 Latin square design,to determine the effects of the basal diets supplemented with 0，8，15 g/d niac in each cow as control group A，group B,group C on performance,maintenance behavior and ion chroma of serum．The feeding experiment had been carried out for 63 days.The results showed :① compared with control group A，milk yield of group B and C was increased by 29.9% (P<0.05)，22.0% (P<0.05),respectively，and 4% fat corrected milk yield was increased by 28.5% (P<0.05)，21.5%(P<0.05),respectively，there was no significant difference between group B and C (P>0.05)；dietary niacin had no effect on the content of milk fat, milk protein and milk sugar; ②compared with control group， there was no effect on the hours of standing and lying, the number of micturition and defecation, the number of drinking-water,but rumination time was significantly increased (P<0.05). Also, there was no significant difference between group B and group C (P>0.05); ③compared with control group A， the Ca2+,K+,Na+ chroma of serum of group B and group C was increased in diffferent extent, but there was no significant difference between group B and group C (P>0.05 ).The Mg2+ chroma of serum was increased,and the Cl- chroma of serum was decreased, but not reached significant levels (P>0.05). As performance,maintenance behavior and ion chroma of serum were concerned,niacin supplementation with 8 g/d in the ration of lactating cows had the best effect and released the effect of heat stress in hot environment．

Key words: niacin; heat stress; dairy cows