Ship ballast water affects various marine species in a port or in a coastal area. The International Maritime Organization (IMO) adopted "the Convention on Ballast Water Management" in February 2004 and insisted that all ships be equipped with a ballast water treatment system beginning 2010. As basic study on a low power consumption ballast water treatment system (BWTS), the disinfection characteristics of $Tetraselmis$ sp. were analyzed using an ultraviolet (UV) light-emitting diode (LED). UV LED modules with peak wavelengths of 255 nm, 265 nm, and 280 nm were fabricated, and each module was exposed to the same UV dose of 100-400 $mJ/cm^2$. The experimental results showed that the highest disinfection wavelength for $Tetraselmis$ sp. appeared at 265 nm with a constant UV dose, and its disinfection rate was 91 %.