A total of 60 of Tilapia nilotica fish brought from El-khashea farm in kafr El-sheikh governorate in plastic cage to Animal Health Institute. Then fish were divided into 3 groups in 3 glass aquarium each one contain 20 fish (Nile tilapia). The fish were fed on the experimental diets during 6 weeks of the experimental period. Fish were fed during the acclimation period on an artificial basal ration at a rate of 3% of the body weight 1 meal daily. Fish in the first group fed on non supplemented control diet while, fish in the second and third groups (A₁, A₂) fed on diets supplemented with 10 & 15% Anabaena azollae respectively. The control, first and second experimental groups (A₁, A₂) was infected with A.hydrophila by using concentrate 1x10⁶cfu/gm. This experiment was designed to study the effect of Anabaena azollae on fish quality, chemical composition of fish including moisture, crude protein, carbohydrates, fat and ash as well as its antibacterial action on A.hydrophila. In control group the bacterial count of A.hydrophila was ranged from 2×10⁵ to 4×10⁵ with mean value 3×10⁵±1×10⁴ cfu/gm, In group (A₁) the count of A.hydrophila was ranged from 1×10⁴ to 2×10⁵ with mean value 2×10⁴±1×10⁴ cfu/gm, In group (A₂) the count of A.hydrophila was ranged from 1×10² to 1×10⁴ with mean value 2×10³±1×10³ cfu/gm. In vitro sensitivity test showed that the anabaena has antibacterial effect on A.hydrophila by 2 concentrations 10% and 15% with inhibition zone 0.8-0.9 mm respectively. Chemical composition of fish in the control group showed mean values for carbohydrates, protein, fat, ash and moisture as 0.9+0.07, 19.9+0.3, 2.9+0.14, 1.8+0.09 and 73.25+0.2% respectively. The mean values of carbohydrates, protein, fat, ash and moisture in first group (A₁) were 0.76+0.09, 21.3+0.3, 2.68+0.1, 1.58+0.16 and 72.9+0.3% respectively. Chemical composition percentage (%) of fish in the second group (A₂) were 0.73+0.07, 21.5+0.3, 2.4+0.19, 1.63+0.1 and 72.48+0.31 for carbohydrates, protein, fat, ash and moisture respectively. So, the mean of A.hydrophlila count decrease gradually from control group to group A₁ with Anabeana 10% to group A₂ with Anabeana 15%. In addition the mean of moisture carbohydrate % and fat % decreases gradually from control group to group A₁ to group A₂. While the mean of protein % increase gradually from control group to group A₁ to group A₂. In addition the mean of ash % decrease from control group to group A₁ to group A₂.