Abstract: The species composition,dominant species,and abundance of phytoplankton were investigated in the northern Huanghai Sea between December 30,2006 and January 17,2007.The results showed that 131 species were identified,belonging to 4 phyla with 68 genera,in which 86.3% were Bacillariophyta (53 genera and 113 species) and 12.2% were Pyrrophyta (11 genera and 16 species).The majority of species were neritic and cosmopolitan in temperate coastal seas.The average quantity of phytoplankton was 88.89×10⁴ cells/m³,Bacllariophyta and Pyrrophyta were 86.58×10⁴ cells/m³ and 2.28×10⁴ cells/m³,respectively.The dominant speciesincluded Eucampiazodiacus,Paraliasulcata,Pseudo-nitzschiapungens,Chaetoceros densus,Chaetoceros debilis,Rhizosolenia setigera.In addition,the average diversity index (H') and evenness (J') values were 1.80 and 0.42,respectively.It was found that the coastal waters of southern Liaoning was the area of highest phytoplankton abundance,followed by the coastal waters of northern Shandong and the central area of the northern Huanghai Sea.The horizontal distribution of phytoplankton was determined by the diatom distribution pattern.In comparison with the investigation of Jan.1959,the abundance of phytoplankton reduced nearly 41%(from 150.00×10⁴ cells/m³ to 88.89×10⁴ cells/m³),among which the abundance of diatoms reduced from 148.00×10⁴ cells/m³ to 86.58×10⁴ cells/m³,while the abundance of dinoflagellates increased from 1.25×10⁴ cells/m³ to 2.28×10⁴ cells/m³ (from 0.8% to 2.5%).The community pattern relating to diatoms as the dominant group,followed by dinoflagellates,as well as the phytoplankton abundance,has not significantly changed irrespective of the above differences.