This study was conducted to explore the use of essential oil (EO) in broiler nutrition as a natural growth promoter. Different levels of EO derived from thyme and cinnamon were added to a standard diet to determine its effects on feed intake, live weight gain, feed conversion ratio and blood constituents. Three hundred day-old broiler chicks (Arbor-Acres) were divided into five equal groups and treated as follows: No EO (Control group), 100 ppm EO derived from thyme (group 2), 200 ppm EO derived from thyme (group 3), 100 ppm EO derived from cinnamon (group 4) and 200 ppm EO derived from cinnamon (group 5). The diets were prepared freshly each day. Experiments was carried out for 42 days. Results showed that chicks fed with 200ppm EO derived from thyme and cinnamon had significantly higher (P<0.05) feed intake, body weight gain and feed conversion ratio, followed by chicks fed with 100 ppm EO derived from thyme and cinnamon compared with control group which  showed the lowest performance. Moreover, the chicks fed with ration containing EO derived from thyme and cinnamon had reduced (P<0.05) cholesterol. The total proteins increased significantly (P<0.05) for the groups containing 200 ppm EO derived from thyme and cinnamon. In conclusion, EO could be considered as a potential natural growth promoter for poultry at the level of 200 ppm, depending on the kind of EO derived from herbal plants.

Key words: Cinnamon, thyme, essential oil, performance, serum biochemistry, broilers.