The distribution characteristics of dominant phytoplankton species and their cell abundance and cell volume conversion biomass off the Changjiang Estuary were analyzed based on a comprehensive survey conducted at 67 stations during July,2006.The cell abundance of Diatom was dominant,which mostly consists of Thalassionema nitzschioides and Skeletonema costatum.The cell abundance ranged from 1.42?10⁶ to 448.25?10⁶ cell/m³ and the average value was 90.47?10⁶ cell/m³ for all of the stations.Phytoplankton conversion biomass of Chlorophyll-a was calculated by cell volume measurement.The average Chlorophyll-a biomass was 4.41 mg Chl.a/m³,with a range from 0.02 to 34.08 mg Chl.a/m³.Based on conversion biomass,dominant diatoms were Coscinodiscus jonesianus, Coscinodiscus radiatus Ehrenberg and Thalassionema nitzschioides, and dominant dinoflagellates were Peridinium conicum, Ceratium fusus, Ceratium tripos and Ceratium furca. The trend and magnitude of the conversion biomass distribution were similar to those of the Chlorophyll-a distribution of SeaWiFS monthly ocean color data.This area could be divided into 4 parts according to phytoplankton conversion biomass:the dilute plume area is the highest biomass area; the estuary area and Taiwan Warm Current area are the lowest biomass areas;and the area off Jiangsu Coast current area is the middle biomass area.The area of conversion biomass more than 4.0 mg Chl.a/m³ corresponded well to the salinity contour 28.0 adjacent to the area of Changjiang river plume and areas with low NH₃-N and P.