Abstract: On 1st June 2003, the Three Gorges Reservoir (TGR)began its storage officially, and the water level reached at 135m on 10 June (and at 139 m on 5 November).Three are a lot of tributaries been flooded in the lower reaches, and many baysare therefore formed from that time in the reservoir.We monitored the changes of ecological environment of TGR, and found thatmost of bays was in eutrophic state and emerged several serious algal blooms predominated by dinoflagellate (Peridiniopsis spp.)and diatom (Cyclotella sp.), especially in spring.In case of Xiangxi Bay, the largest one in Hubei part of TGR and the onemost close to the dam, the monthly monitoring of 2 years shown that the trophic state in most month was eutrophic, except inwinter.Farthermore, the trophic state in spring was hyper-eutrophic.On the other hand, to get a general information of eutrophicationsituation in TGR, we assessed the trophic state of 22 bays and TGR itself in spring, 2005.The results shown that 5 bays(22.7%)were in meso-trophic, and 17 bays (77.3%)were in eutrophic (10 bays in hyper-trophic, occupied 45.5%).Thewater quality of TGR itself, however, was still in good condition (meso-trophic), due to its fast exchange rate of water.Statisticanalyses shown that relationship between chlorophyll a and watershed characters (annual discharge and catchment area)of baysin TGR was significant negative.It implied that, generally, the smaller of the bay (or its annual discharge), the higher ofchlorophyll a concentration, i.e.the easier of forming algal bloom in spring.Based on the monitoring and researching, we discussedand proposed several strategies to control ormitigate eutrophication phenomena in TGR, including pollution control, completesurvey, mechanism study and eco-hydraulic regulation.