The research was conducted to investigate the immunomodulatory and growth promoting effects of basil (Ocimum basilicum) and ascorbic acid in heat stressed broiler chicken. A total of 360 (Hubbard) one-day old broilers were reared under heat stress (38 ± 1°C) and randomly divided into three groups i.e. A, B and C as control, basil seed at 5g/kg and ascorbic acid at 200 mg/kg supplementation, respectively. Improvement of intestinal villus seen in basil supplementary group (P>0.05), and significant increase in the absorption of crude protein (CP), crude fibre (CF), and metabolized energy (ME). However, the values of red blood cells and packed cell volume were non-significant whereas white blood cells, haemoglobin, and new castle disease antibody titer were significantly higher in basil supplementary group. The weight gain and feed conversion ratio significantly improved in basil treated group, while ascorbic acid and basil significantly decreased the water intake. Body temperature and mortality percentage were significantly decreased in ascorbic acid and basil supplementary group as compared to control. Dressing percentage significantly increased in both supplementary groups. It is concluded that supplementation of basil at 5g/kg feed, promotes the growth, improves the intestinal villus size, feed efficiency and immunity of heat stressed broiler chicken.