To assess the impacts of grape seed extract (GSE) fed with or without chromium nicotinate (CN) on beef steers under heat stress conditions, 40 Simmental × Qinchuan steers (400 ± 10 days old 410 ± 8.0 kg) were randomly assigned to one of four diets (n = 10 per group): basal diet (CON group) basal diet + 33 mg/day CN (CN group) basal diet + 65 mg/kg DM GSE (GSE group) and basal diet + 33 mg/day CN + 65 mg/kg DM GSE (CN + GSE group). This study was conducted in summer and the steers were housed in outdoor shaded pens (10 steers/pen) and fed individually. The experiment lasted for 35 days, the first 7 days for diet and housing condition adaptation. The amount of feed intake was recorded daily individual bodyweight was recorded on Days 8, 22, and 36. On Day 36 before feeding, six steers per group were slaughtered for collection of blood and tissue samples. Average daily liveweight gain and dry matter intake of steers were increased by the GSE, CN, and CN + GSE treatments (P < 0.05). Dietary treatments increased the activities of glutathione peroxidase and total superoxide dismutase in plasma (P < 0.05) and plasma concentration of interleukin 10 on Days 22 and 36 (P < 0.05), whereas decreased plasma concentration of tumour necrosis factor-α on Day 22 (P < 0.05). Overall, supplementation of GSE alone or with CN had positive effects on the growth performance of steers under heat stress conditions.