Six hundred and forty O. niloticus were equally distributed in 16 hapas (each of 1.5 m3) to be used for 4 treatments in 4 replicates. Fish of groups 1-3 were fed on basal diet mixed with Echnicacea (0.25 ppt) for 1 st, 2 nd and 3 rd summer months; respectively (1st phase). Fish of group 4 were fed basal diet only as a control. By the end of the 3rd months, some fish were used to measure hematological, immunological parameters. The remaining fish in the 4 hapas of each treated group were fed with the basal diet without Echinacea for 6 months (winter season, 2nd phase). The same growth parameters, survival and relative protection against challenge infection were determined by the end of 1st and 2nd phase of the experiment. At end of 1st phase, a marked increase in the neutrophil adherence, hematocrit values and total leukocytic count was seen in most treated groups. Also an increase in the body gain, specific growth and survival rates were recorded. At end of 2nd phase, no significant changes were seen in the body gain, specific growth rate and coefficient factor but a significant increase in the survival rate was noticed. Challenge of experimented tilapia resulted in low mortality rate of all treated groups in comparison with the control. The level of protection was high in both seasons; however it was higher in winter than summer season. As a general observation, one month administration of 0.25 ppt of Echinacea induced remarkable effect on the survival and protect against challenge infection. The prolonged addition of 0.25 ppt Echinacea (2 & 3 months) showed similar effect on survival and more protection against challenge.