Abstract: It is reported in this paper that the results of allolepathy test of P. pectinatus L on Scenedesmus obliquus(Chlorophyta)and Microcystis aeruginosa(Cyanobacteria)with the filtrate of liquious cultured the macrophyte. Both macrophytes and phytoplankton are primary producer,and play the similar key role for material cycle and energy flow in aquatic ecosystem. Although there are many reports on the competition of nutrient and light between both of them,the biochemical interference of secreting substance from these primary producer is another important thing. Now the allelopathy of macrophytes on algae were paid more and more attention,by the reason how to control the pollution of cyanobacterial bloom. For the first time,we try to illustrate the qualitatively ecological significance of the allelopathy with bioassay approaches to calculate the allolepathic effect of biomass(Eb),of growth rate effect(Er)and the coefficient(Et).