Abstract: A co-culture system of Nymphoides peltatum and natural frond dominated with blooma bluegreen algae (Microcystis aeruginosa) was established for analysis of variation of cell density and content of chlorophyll-a of various algae, growth of Nymphoides peltatum and underwater light intensity in the system. Meanwhile,M.aeruginosa was cultured in the medium prepared out of the culture solution of N. peltatum under a proper light condition for analysis of variation of the physiological indexes of alga growth with time. Results show that total algal cell density dropped significantly for all the algae in the system(P<0.05), and that of M. aeruginosa in particular by 94.68% as compared with the initial density 40 days before. However, the population of Tetraedron minimum was growing gradually to be a dominant species. It is obvious that the growth of M. aeruginosa was significantly inhibited by N. peltatum.In the co-culture system, chlorophyll-a of algae was in significant negative correlation with fresh weight of N. peltatum and light attenuation rate at 20 cm underwater,indicating that photosynthetic capacity of the algal cells is closely related to the growth of N. peltatum and its shading effect. With the culture going on,light density(D₆₅₀) of M. aeruginosa cells decreased significantly,and so did the relative content of chlorophyll-a and phycobiliproteins (including phycocyanin, allophycocyanin, phycoerythrin) by a varying degrees or down to 5.27%,15.53%,21.11% and 48.48%, respectively after 9 days of incubation of the system and Ca²⁺ Mg²⁺ -ATPase activity as well. All these findings suggest existence of some active substances that have allelopathic effect on M. aeruginosa in the N. peltatum culture solution and affect the photoreaction system (PS I and PS Ⅱ) of M. aeruginosa,inhibiting photosynthesis and growth of M. aeruginosa.It is quite obvious that besides the shading effects, allelopathic active substances secreted by N. peltatum is also an important mechanism of N. peltatum inhibiting growth of M. aeruginosa.