Abstract: Based on the maritime data collected from 23°30′–33°00′ N and 118°30′–128°00′ E of the East China Sea in four seasons during 1997–2000, we analyzed the dynamics of medusa diversity and the causes. A total of 103 medusa species were observed, which were mainly distributed in southern and northern offshore of the East China Sea. Higher species diversity indices (H′) of medusa occurred, respectively, in the southern part of the sea, offshore, and in summer and winter. The number of species was closely correlated with H′ value, whereas the abundance was not correlated with it significantly. The lower H′ value nearshore in spring and autumn resulted from the aggregation of Muggiaea atlantica nearshore of Zhejiang Province and Diphyes chamissonis at the Yangtze River estuary. Water temperature, followed by salinity, was main environmental factor influencing the distribution of species diversity. H′ value was related to the water temperature of 10 m layer in winter and spring, while it is associated with surface water temperature in summer and with 10 m-salinity-layer in autumn. The isoline distribution of H′ value reflected the direction of currents and changes in water masses in the East China Sea, the H′ isoline was a good indicator for analyzing the East Sea cold masses.