The effect of green synthesized nanoparticles on fish larvae pathogenic bacteria has so far been rarely investigated. In the present study, silver nanoparticles “AgNPs" were synthesized biochemically from the reduction of AgNO₃ by leaves extract of mangrove (Avicennia marina) plants, and their antibacterial activity against fish larvae pathogens was assayed. To define the characterization of the green synthesized AgNPs, UV-Visible spectra, IR (infra-red), X-ray powder diffraction (XRD) and photoelectron spectroscopy (XPS), high resolution transmitted electron microscope (HRTEM), and selected area electron diffraction (SAED), were utilized. Results determined the formation of nano-size AgNPs with ranges 15 to 25 nm. The isolated bacteria, from fish larvae (sardines, pipefishes, cardinalfishes, dragonets, silversides, and wormfishes), were identified by morphological characterization and biochemical tests. Thus, the isolated bacteria was suggested to belong to Vibrio spp. (Vibrio alginolyticus, Vibrio vulnifacus, Vibrio parahaemolyticus, Vibrio harveyi, and Vibrio cholera). Except for A. marina leaves extract, the green synthesized AgNPs, with an inhibition zone radius of ~13-20 mm, showed an effective impact on all those pathogenic bacteria.