Abstract: The investigation on phytoplankton of abundance,dominant species was carried out from May 2003 to May 2004 in Ji2hongtan Reservoir,Shandong Province1Jihongtan Reservoir is the largest plain reservoir in China. We sampled phytoplankton ininlet and outlet of the reservoir every month1The samples were identified and counted with microscope1The results showed thatthere were 63 species in total,representing 49 genera1And there was no significant difference among the inlet, outlet and centreof Jihongtan Reservoir with phytoplankton composition1When biomass was concerned,inlet > centre > outlet,which means thereservoir’s ability to self purify was working1The phytoplankton composition changed significantly during the year1The phyto2plankton was dominated by diatoms and cryptophytes in winter and spring,the species respectively were Cylotella sp1andChroomonas acuta, but in summer and autumn were cyanobacterias and green alga1The standing corp of phytoplankton peakedonly in autumn1The average phytoplankton fresh biomass was 1182 mg/L(inlet) and 1109 mg/L(outlet),ranged from 0134 to7177 mg/L1The lowest value was appeared in outlet inJanuary1Cyclotella sp1was 6916 %to total phytoplankton biomass,and thehighest value appeared in inlet in August,filamentous cyanobacteria was 71144 % of the total phytoplankton biomass1In summerwe found some Microcystis sp1colony on the surface of the reservoir,even in winter some of them were still there. The averagevalue of Shannon2Weaver diversity indexof phytoplankton community was 1188, rangedfrom0193～3130,the lowest and highestvalue appeared in inlet in January 2004 and centre of reservoir in November 2003,respectively1The diversity index of inlet wasaffected lightly by indraught, but the outlet was relatively stable1In general, the diversity indexesof outlet were higher than inlet,but not significantly1In conclusion water quality was well, belong to mesotrophic,but had the potential to turn to eutrophica2tion1So we should pay attention to protect the water source and enhance the management of the reservoir1For example we can in2draught water when water quality was well and raise some herbivore fish in order to control the growth of phytoplankton1