A total of 800 one-day-old chickens (Cobb 500) were obtained from a local hatchery and allocated randomly to five dietary treatments. This study conducted for 42 d. Treatment groups included 1) CON (control), 2) T0.3 (0.3% of thyme extract), 3) T0.6 (0.6% of thyme extract), 4) G0.3 (0.3% of garlic extract) and 5) G0.6 (0.6% of garlic extract). Thyme extract included $90{mu}g/ml$ Carvacrol and 400 mg/ml thymol while garlic extract included 100 mg/ml elastin. There were no significant effects among the treatments on weight gain, feed intake and feed conversion ratio over the entire trial. Also, no significant differences were observed between CON group and other treatments in plasma parameters (cholesterol, triglyceride and HDL) and hematocrit. Sex of chickens had no effect on blood parameters. Supplementation of herb extracts did not affect carcass characteristics including carcass yield, breast, thigh and abdominal fat. There were no significant differences observed in carcass characteristics between males and females fed with different levels of herb extracts. In conclusion, supplementation of garlic and thyme extracts as feed additives at two levels of 3 and 6% in the present study did not improve performance and carcass traits of broiler chicks.