The objective of this study was to investigate the effects of thyme and oregano essential oils (as growth promotors), individually and in combination, on the health, growth performance, and prevalence of muscle abnormalities in broiler chickens. Six hundred day-old Cobb 500 hybrid chickens were randomized into four dietary treatment groups with three replicates each. Chicks in the control group (C) received a basal diet, while the experimental treatment groups received basal diets containing 350 mg/kg of thyme oil (T1), 350 mg/kg of oregano oil (T2), and 350 mg/kg of thyme and oregano oil (T3). Growth performance parameters were evaluated at 14, 28, and 42 days. The broilers in treatments T1 and T2 had significantly higher body weights than the control group. The feed conversion ratio was the lowest in chicks who received oregano oil, followed by those fed thyme oil. The overall prevalence of growth-related breast muscle abnormalities (including white striping and white striping combined with wooden breast) in groups receiving essential oils (T1, T2, and T3) was significantly higher than in the control group (C). The thyme and oregano oil diets showed no significant differences in antibody titers against Newcastle disease or interferon-γ (INF-γ) serum levels. In conclusion, thyme and oregano oils had a positive impact on the growth performance of broiler chickens but increased the incidence of growth-related breast muscle abnormalities.