Shading of phytoplankton on submersed macrophytes in eutrophic waters is considered to be one of the main causes for the decline of the macrophytes. However, growth inhibition of submersed macrophyte by phytoplankton may be also due to other important factors including alleloph and DIC competition of phytoplanktion. In order to study whether or not shading is the major effect of phytoplankton inhibiting submerged macrophytes, a laboratory experiment was carried out with a submersed macrophyte Vallisneria natans grown in dense phytoplankton water. Similar shade levels were created either by dark fabric screens or by phytoplankton dominated by chlorophyta (irradiation in the water was 10%-33% that in the air). Growth indices of both number and length of leaves, and biomass of V. natans were measured during the experiment. Chlorophyll a + b concentration and net photosynthesis rate in leaves were also monitored. Growth and photosynthesis of V. natans under the shade screens were significantly higher than the plants grown with the chlorophyta, even the leave tips of the plant were found decay in later treatment. The results indicated that chlorophyta suppressed the growth of V. natans in ways other than shading. Allelopathic inhibition of chlorophyta on the net photosynthesis rate of V. natans, competitive advantages for photosynthetic active radiation ( PAR) and dissolved inorganic carbon ( DIC) of chlorophyta over V. natans were suggested.