The objective of the present study was to ascertain the influence of thyme leaf powder (TLP) on growth rate, carcass features, and blood indicators in broiler diets as a natural antioxidant and antibiotic substitute. A fully randomized design experiment divided 264-day-old broiler chicks (Ross 308) into 4 experimental groups. Six replicates of each group, each containing 11 unsexed chicks, were created. The following were the therapies: control group without additive (basal diet); TLP1, TLP2 and TLP3: basal diet + 1, 2 and 3 g thyme leaves powder/kg diet, respectively. Results showed that TLP added at 2 or 3 g/kg of feed significantly enhanced body weight gain (except for the period from 16 to 30 d of life), feed intake (except for the period from 1 to 15 d of life), and feed conversion ratio throughout study period. Furthermore, there was a significant decrease in creatinine and alanine aminotransferase (ALT) with different TLP levels, particularly at a 3 g TLP/kg diet. Compared with the control group, the birds' 2 g TLP/kg diet included the highest levels of albumin and total protein and A/G (albumin/globulin) ratio. Furthermore, the birds fed 1 g TLP/kg feed had the lowest concentrations of low-density lipoprotein (LDL) and total cholesterol (TC). Moreover, chicks fed all treatment diets containing TLP had higher IgY concentrations than the control group. Nevertheless, the group that was fed a diet including 2 g TLP/kg had the highest IgM levels. Additionally, the superoxide dismutase activity (SOD) levels were highest in the birds fed a 1g TLP/kg diet and minimum in the birds fed the control diet. Notably, malondialdehyde (MDA) levels were maximum in birds fed a control meal and lowest in birds with a diet containing 1 g of TLP/kg. We may conclude that using TLP in broiler diets increases the birds' immunity, productivity, and overall health, especially at level 2 g of TLP/kg diet.