The effect of dietary inclusion ِِِِِlevels of  Origanum vulgare as a feed additive compared with a control diet for 13 weeks on growth performance, feed utilization, whole body composition, entropathogenic Aeromonas hydrophila-challenge and economic efficiency for production of one Kg gain of Nile tilapia Oreochromi niloticus were studied. All experimental diets were iso-nitrogenous (30% crude protein) and iso-caloric (4.42 k cal/g diet) with or without Origanum vulgare addition. Fifteen glass aquaria were used, each one was stoked with 15 fish (1.88±0.01g/ fish) and three aquaria were designated for each treatment. The five treatments were: treatment 1 (was a control diet without additives), and four treatments (T2,T3,T4,T5) contained 0.5 %, 1.0 %,1.5 % and 2.0 % of Origanum vulgare respectively . Fish were fed on one of these experimental diets at feeding level of 8% of fish body weight for the first four weeks then reduced to 5% for the next four weeks and then reduced to 3% for the remaining period of the experimental .The feed offered twice daily for six days per week. The results of this study revealed that, fish growth {indicated as final weight, weight gain, daily gain, relative growth rate (RGR), and specific growth rate (SGR)} was increased gradually and the maximum growth was obtained when fish fed the diet contained 1 % of Origanum vulgarwhile the lowest growth was obtained from the control group. The highest feed intake and the best feed conversion ratio (FCR) were obtained when fish fed at 1% of Origanum vulgar.  The lowest feed conversion ratio (FCR) was obtained of the control diet, and the highest feed efficiency ratio (FER),protein efficiency ratio (PER), apparent protein utilization (APU), and energy utilization (EU) were enhanced significantly when fish fed diet contained 1 % of Origanum vulgare compered with control diet. Moreover, Dry matter (DM) and crude protein (CP)were similarly at all the experimental groups , but, the  highest crude fat (CF)was obtained from the diet contained 0.5 % of Origanum vulgar( T3) and the lowest (CF) was obtained from fish fed diet contained 1 % of Origanum vulgare (T4). The highest reduction in feed cost compared with control diet showed to produce one kg fish gain of treatment containing 1 % Origanum. The results of fish challenge against A. hydrophila for 10 days, mortality percentage did not observe in fish fed with diet containing different levels of Origanum, while mortality percentage in fish of control group was 90 %.This study clearly showed that the optimum level of Origanum vulgarein a practical Nile tilapia diet is 1%.