The ballast water of oceanic ships is the main cause for worldwide transfer of non-indigenous aquatic species. The use of hydroxyl radicals is an efficient method for killing the aquatic species in ballast water. For practical application, a 250 t/h center dot OH ballast water-treatment system was designed and installed on the ship Yulong (ten thousand tons) in Dalian harbor. On this ship, a series of experiments were carried out. As a result, the concentration of total oxidants declined over time. Harmful aquatic organisms and pathogens in the ballast water were rapidly killed after center dot OH treatment and no re-growth was observed during a storage time of 5 days. The quality of the ballast water was considerably improved. Therefore, the D-2 ballast water discharge standard of the International Maritime Organization was satisfied. Compared with current methods, center dot OH treatment is a potentially effective technology which can be practically applied in the ballast water-treatment of oceanic ships in the future.