Aggregate of 180 Cobb broiler chicks were randomly assigned to 5 dietary groups, each group contain three replicates (12 chicks each) the experiment lasted 38 days. The first group fed on basal control diet without any additives, while, the rest fed on basal control diet supplemented with 1.0, 1.5, 2.0 and 2.5% marjoram powder (MP) respectively. The obtained results showed that dietary MP improved performance parameters of broilers especially with 1.5% MP, whereas causing significant increase in live body weight at 21 and 38 days of age, body weight gain, improving feed intake and feed conversion ratio at all periods than the control group and other treatments. Red and white blood cells, hemoglobin, alanine aminotransferase, aspartate aminotransferase, total protein and albumen were influenced by MP levels, but platelets, globulin and albumen to globulin ratio were not affected by treatments. Marjoram powder supplementation decreased serum cholesterol level, triglyceride and low density lipoproteins, but increasing high density lipoproteins and glutathione peroxidase activity compared to the control group. Overall, these data indicate that, supplementing MP into the broiler diets improved productive performance, physiological parameters, antioxidant enzyme activity (glutathione peroxidase) and lipid profiles especially with 1.5% MP.