Abstract: In order to study the planktonic algae diversity and its water quality monitoring in Chai River,Baoxiang River and Panlong River,which are the three main and important inflow rivers of Dianchi Lake,located southeast and north of Dianchi Lake in Kunming City in China.In May,2011,after field survey and sampling planktonic algae from 24 sites from upstream to downstream,the rivers were studied by the physical and chemical index analysis,the biological indicator method and biological diversity index (Maglarf,Shanon-Wiener and Simpson diversity index) .The results showed that the river of phytoplankton diversity and various physical and chemical indexes value didnt change sequentially from low to high according from upstream to downstream .The results showed that the algae could be examined in the rivers when TP quantities were more than 0.1 mg/L and TN quantities were more than 0.8 mg/L in the sample water.The algae was indicated in 10 samples,and the algae that the frequency of them was above 12.5% mainly was Navicula,Synedra,Cyclotella of Bacillariophyta,Scenedesmus,Ankistrodesmus,Closterium of Chlorophyta,Euglena of Euglenophyta.Microcystis was the main genera of the water and Navicula,Scenedesmus,Ankistrodesmus,Euglena,Cyclotella existed in all the three rivers,they were the main indicator species of polluted water.The planktonic algae detection rate was 33.3% in Panlong River,which was the lowest in the three rivers,was 50% in Baoxiang River, and was 80% in Chai River.The amount of algae in Baoxiang River and Chai River were far more than that in Panlong River,which mean that the water quality of Panlong River was obviously improved in recent years.We thought that the planktonic algae detection rate as a river water quality monitoring index could reflect dynamically the whole condition of the river water quality to some extent.In the few sample case,the Shannon index was more suitable than other indexes to the river water quality monitoring and assessment.