A 35-d trial was conducted to study the effect of phytogenic additives on growth performance and cecal bacterial populations of broilers. The feeding program consisted of a starter diet until 21-d and a grower diet until 35-d. There were 4 treatment groups: control, 50g EO (essential oil)/ ton, 100g EO/ton and 150g EO/ton. Slight non significant improvements have been observed for body weight, weight gain, feed intake and feed conversion ratio in the starter stage when birds fed diets contained 50g EO/ton. In both grower stage and through out the entire period of growth study no differences in growth parameters have been observed when birds fed diets contained 100g EO/ton and those fed the control diet. The worst performance has been observed by the birds fed diets contained 150 EO/ton through the whole stages. Yet, in general, there were no significant difference in growth performance between birds fed the control diet and those fed diets supplemented with EO.  No mortality occurred during the whole period of the growth study. Using a commercial blend from thymol and trans-cinnamaldehyde revealed a disparate impact on cecal microflora population. Yet, this was not reflected on the bird growth performance. This may be referred to the balanced diet and controlled condition under which the trial was conducted. The present data suggest, further investigation should focus on the mechanism of EO, the ratio between components of the blend and their effect on bird performance under uncontrolled conditions such as (heat stress, unbalanced diet …..etc.) by increasing levels of EO.