The purpose of the current study was to examine the effects of dietary supplements of Holstein dairy cows with rumen-protected niacin on colostrum quality, milk yield and composition, some blood biochemical parameters, antioxidant status, immunity and incidence of some metabolic disorders during the summer season. A total of 50 pregnant multiparous lactating Holstein cows (3 ± 1 parity) with average body weights of 650± 15 kg. The experiment was done from the last month of pregnancy and continued for four months after calving during the summer season (starting from June to September). The cows were assigned randomly into two experimental groups (25 cows each), as control and treated and fed the same TMR (total mixed ration) that was formulated to meet cows' requirements (NRC, 2001). All cows were fed similarly, except the treated group received 5 gm of rumen-protected niacin (NiaShure™) / head/ day. Values of blood biochemical parameters showed a significant (p < 0.05) rise in glucose, albumin and lysozyme levels for the niacin group, compared to the control during July and August. In contrast, Malondialdehyde (MDA) values showed a significant (p < 0.05) decrease during July, August and September, while haptoglobins and Oxidative Hemolysis of Peroxidase-Treated Red Blood Cells (OHdG-8) recorded a significant (p < 0.05) decrease during July and August only. However, there were non-significant differences between the two groups for dry matter intake (DMI), milk yield, milk fat, protein and solids non fat values. However, the niacin group noted a significant decrease in % of metabolic disorders accompanied by parturition, such as endometritis, mastitis, ketosis and retained placenta, in addition to improvement of colostrum quality and quantity during all summer season. The findings suggested that niacin supplementation could potentially play a role in alleviating the adverse effects of heat stress on lactating animals.