Allelopathic effects of C.alternifolius(Cyperus alternifolius),M.aquaticum(Myriophyllum aquaticum),C.alternifolius and M.aquaticum were investigated in coexistance experiments,and the algae biomass,chlorophyll a,community change and organic matters of the coexistance samples were monitored and analyzed.Cell numbers and Chl-a were significantly inhibited by C.alternifolius and M.aquaticum.The peak biomass of algae is 2.4×107/L,while that of the control,C.alternifolius,M.aquaticum is 4.48,1.15,2.03×108/L respectively.The organic matters analysis show that only 75 kinds of organic matter exist in the C.alternifolius and M.aquaticum coexistance samples while 131 kinds of organic matter exist in the control sample.Acetone cyanohydrin,Triethyl phosphate,Camphene and 4-methyl-4-Phenol were new produced matters by C.alternifolius and M.aquaticum which maybe the potential allelopathic matters.