Experixnental studies were carried out on the top-down effects of silver carp (Hypophlhalmichthys molilrix) and bighead carp (Amtichthys nobilis) at a stocking level of 15g/m3 in freshwater microcosms under relatively high nutrient loading (average P loading=0.0061 g P/(m³·day)). The results indicate that the community structure, metabolism and physico-chemical conditions of the microcosms stocked with fishes changed obviously; the microcosms containing bighead carp alone (monoculture) or both species of fish (mixed culture) became more or less eutrophic at the end of the experiment. It is found that primary production was correlated positively with phytoplankton density but negatively with zooplankton density in the microcosms, so the differences in productivity among various groups of microcosms could be on the whole explained by the trophic cascade hypothesis. However, the dynamics of plankton community show both top-down and bottom-up effects between zooplankton and phytoplankton, and the increase in algal density did not entirely result from the trophic cascade effects. It is considered that dense stocking of silver carp and bighead carp for the purpose of increasing fish yield played an important role in accelerating the eutrophication processes of natural waters with rich nutrient sources.