In the survey area, there are four maxima and two clear minima of seasonal variation in total numbers of individuals of hydromedusae a year. The change of locations in the dense center of hydromedusae shows that it moves northward from southwest in spring and summer, and it retracts from north to southwestern part of the strait in autumn and winter. The seasonal variation in number of species of hydromedusae may be divided into two types: ( i ) species occur throughout the year. (ii) species with markedly seasonal variation. There are three dominant species in the survey area, i. e. Liriope tetraphylla, Aglaura hemistoma and Rhopalonema velatum. The former two are of oceanic eurytopic type, the last one is of oceanic steurotopic type. The hydromedusae in investigated area may be divided into three ecological groups, i. e. neritic warm-water, oceanic warm-water and neritic warm-temperal, the former two of which are dominated. In the survey area, there are four maxima and two clear minima of seasonal variation in total numbers of individuals of hydromedusae a year. The change of locations in the dense center of hydromedusae shows that it moves northward from southwest in spring and summer, and it retracts from north to southwestern part of the strait in autumn and winter. The seasonal variation in number of species of hydromedusae may be divided into two types: (i) species occur throughout the year. (ii) species with markedly seasonal variation. three dominant species in the survey area, ie Liriope tetraphylla, Aglaura hemistoma and Rhopalonema velatum. The former two are of oceanic eurytopic type, the last one is oceanic steurotopic type. The hydromedusae in investigated area may be divided into three ecological groups, ie neritic warm-water, oceanic warm-water and neritic warm-temperal, the former two of which are dominated.