AbstractMacrobenthic mollusk is one of the common marine benthic groups in the Jiaozhou Bay, which plays a very important role in food chains in the area. There has been much ecological research on macrobenthos of Jiaozhou Bay, but few studies on the species diversity of macrobenthic mollusk in detail . Based on the data collected from ten sampling stations from Febuary 1998 to November 2004, we studied the species diversity of macrobenthic mollusk fauna. Sampling was conducted in spring (May), summer (August), autumn (November), and winter (February), using a 0.1 m² grab and then preserving samples in 75% alcohol. We identified, counted, and weighed the samples. Because of the large-scale aquaculture of Ruditapes philippinarum on the edge of the bay, it was difficult to distinguish natural populations from cultured ones, so this species was not included in the present study. A total of 62 species were found, of which the dominant species were Theora lubrica and Nucula faba. Common species were Moerella jedoensis, Raetellops pul-chella, Eocylichna cylindrella, Nassarius succinctus, Cycladicama tsuchii, and Moerella iridescens. The Shannon-Wiener index (H′) showed no remarkable fluctuation during 1998 to 2004, but demonstrated dis-tinct seasonal variations: the average H' indices were higher in autumn and winter than in spring and summer. Pielou’s evenness index (J) peaked in autumn (0.87), and was lowest in summer (0.58). The species richness index (D) was relatively lower in summer (1.82), but no remarkable difference existed in other seasons.