Seasonal and inter-annual changes in species composition and abundance of jellyfishes and their relationship with environmental factors in Jiaozhou Bay were studied based on the data collected from 1980 to 1998. The seasonal variation in abundance shows that there were generally two peaks. The first one, usually the highest, occurs in spring and the second occurs in summer. Water temperature seems to be the main factor which controls the seasonal variation in abundance of jellyfishes. The density of jellyfishes inside the bay was higher than that of outside the bay from March to August, but the situation was opposite from October to February next year. September is the transition month. Their horizontal distribution was affected by salinity when salinity was higher than 32‰. The jellyfishes density decreased obviously as salinity increased, since most of the jellyfishes species in Jiaozhou Bay are neritic. There seems to be a trend that the bio-diversity and abundance of jellyfishes in Jiaozhou Bay decreased from 1980’s to 1990’s.