Coated and uncoated formalized killed vaccine of Aeromonas hydrophila (bacterin) were used in the immunization of Nile tilapia against Motile Aeromonad septicemia. Vaccination was conducted via oral route.Fish were fed on oral initial dose followed by 2 booster doses every 2 weeks at 2^nd and 4^thweek . Blood samples were taken at zero day to get the pre-immune serum and every two weeks 2, 4 and 6 weeks post immunization from the initial dose and  at 8^th week post challenge for antibody titration in the fish sera. The humoral antibody response of vaccinated Nile tilapia (Oreochromis niloticus) was determined by direct agglutination titer test. Moreover, lysozyme activity was assayed for non-specific immunity. Experimental challenge infection with the virulent strain of Aeromonas hydrophila was done and daily mortality was recorded for 14 days to estimate the cumulative mortality and relative percent survival (RPS) of the fish in both control and vaccinated groups. The results of the group that fed on coated formalized killed vaccine gave the highest Antibody titers, lysozyme values, and RPS percent.