Abstract: Based on the data from historical field survey and recent ecological monitoring,biodiversity of the main marine species and its long-term variations were studied in the Laizhou Bay.Phytoplankton,zooplankton,and benthos were analyzed respectively.The results showed that phytoplankton species in the Laizhou Bay was of a wide variety from tens to more than a hundred in different cruises.Both annual and inter-annual variations of the phytoplankton dominant species were characterized by community succession,and the total phytoplankton abundance varied exponentially.The diversity index H' (log2) of phytoplankton community averaged 2.64.The mean H' in autumn was higher than that in spring and summer,while the mean H' in 1959 was much higher than that after 1980s.The main zooplankton group was copepods,and the species number of zooplankton community was lower than that of phytoplankton.The composition of dominant zooplankton species was relatively stable,and the individual density had a decreasing trend in recent years.The diversity index H' of zooplankton community was at a range of 1.74~2.52.The benthic species in the Laizhou Bay was numerous,and the species number of only macrobenthos could reach more than a hundred in a single cruise.The dominant macrobenthic species were mainly polychaetes and bivalves.In recent years,the macrobenthos were characterized by high abundance and low biomass,and the diversity index H' had an increasing trend but still low relatively to that of 1980s.