We characterized the phytoplankton assemblages in ballast water from 25 vessels and sampled at 4 ports (Incheon, Kwangyang, Ulsan and Pusan) on the korea from April to December 2007. Phytoplankton samples collected from each vessel were fixed by lugols solution, and identified using a light microscopy. The number of phytoplankton species varied from 3 to 45 including 5 potentially harmful species. Bacillariophyceae was prominent compared to the rest of classes once the salinity of ballast water was higher than 27 psu. The habitants of fresh and brackish waters, such as Pediastrum sp. dominated ballast water samples taken from HOHEWEG and OPALACE which showed 2.4 psu and 0.6 psu respectively. Most phytoplankton identified were commonly found species in the coast and off shore of South Korea as most ship's ballast water were loaded from geographically neighboring countries around Asia. Therefore, further investigation of ship's ballast water from other continents is required to study the issues.