Abstract: An epidemic disease named “milky disease” happened in cultured swimming crab Portunus trituberculatus from July to September during theyear 2004 in Zhoushan, Zhejiang Province and lead to high mortality. The disease crab is usually thin and has the symptom of muscle white, milky liquid can be seen when open the lid..No predominance bacteria colonies were isolated from the typical milky crab.The study of the histopathology showed that the haemolymph in the disease crab decreased sharply, and was replaced by large amount of parasites, which was observed in muscle,heart and hepatopancreas from the disease crab. The pathological characteristics mainly appeared as cells swelling, necrosis, nucleus condensation and collaspe.The opalescen hemolymph and abnormal tissues were observed under electron microscope, masses of the parasite were observed but no virosome had been found.The parasite is 5-7μ m in size, and oval in shapes,with two flagellums.Preliminary study showed that the dinoflagellate Hematodinium. sp was the main pathogen of the milky disease of Portunus trituberculatus.