This paper dealt with the disinfection performance by Ultra-Violet Light Emitting Diode (UV LED) for a phytoplankton as a basic study for the development of a low-energy consumption ballast water treatment system. UV LEDs having peak wavelength of 255nm, 265nm and 280nm were used in the experiment. UV LED modules with driving circuit were fabricated, and optical and electrical characteristics of them were analyzed. The disinfection performance for phytoplankton depending on the UV wavelength was evaluated by comparing the number of phytoplankton before and after the UV treatments. The experimental result showed that the highest disinfection wavelength for the phytoplankton was 265nm.