To confirm whether or not the sodium hypochlorite meet with the D-2 regulation established by IMO (International Maritime Organization), the biological efficacy was tested using marine organisms which consisted of three groups such as 10-50㎛ (mainly phytoplankton), ≥50㎛ (zooplankton) and bacteria. Experimental conditions were 5ppm and 10ppm for final chlorine concentration in the sodium hypochlorite treated tanks. The efficacy of the phytoplankton was analyzed by three kinds of measurements with photomicroscope, epifluorescence microscope and fluorometer (Turner Designs 10－AU). After treatment of sodium hypochlorite, all motile phytoplankton lost their motility, the color of chlorophyll turned from red into green, and the chlorophyll fluorescence changed into zero value in both concentration of 5ppm and 10ppm. These results indicated the phytoplankton was totally killed after treatment. Survival for zooplankton was determined based on the motility by touching it gently under a stereo microscope. Natural assemblage collected from the treated tank was immediately killed shortly after treatment of sodium hypochlorite. The target bacteria in the treated tank were aerobes, coliform and E.coli which were completely killed after treatment on the basis of inoculation method using Petrifilm plates (3M TM). These results indicate that, if NaOCl is introduced into waters including marine plankton, bacteria, phytoplankton and zooplankton can be effectively controlled.