A pure basil oil which have antimicrobial and immunostimulant effect in the cultured fish was formulated into nanoemulsion using homogenizer. Basil oil nanoemulsion characters, its effect as antimicrobial in vitro and its resistant against Aeromonas hydrophila were studied. The results revealed that the droplet size distribution of the basil oil nanoemulsion measured 6.419 nm in diameter with poly dispersity index(PDI) value 0.753, Zeta potential -1.01mV, pH value 5.55, the conductivity 203.5 µS/m and the viscosity 0.887 mpascal/sec. Basil oil nanoemulsion had antibacterial effect at 6µl concentration against Aeromonas hydrophila, Aeromonas veronii, Pseudomonas fluorescensand Streptococcus agalactiae while, at 3µl concentration had antifungal effect against Saprolegniaparasitica.Minimum inhibitory concentration (MIC) of this formulation of basil oil differ among the examined bacterial isolated from 3-9 µl. Basil oil nanoemulsion save for O. niloticus at 8.5% of fish body weight for 14 days and increased fish resistance. Moreover, basil oil showed different effects against A. hydrophila with using ultrastructure electron microscope were discussed in this study.