Spatial-temporal variation trends of Chlorophyll-a concentration in Lake Dianchi were analyzed based on MERIS data.First,three popular retrieval algorithms of Chlorophyll-a concentration were calibrated by field measurements.It was found that three-band model was the best one with higher estimation accuracy.Then, the optimized three-band model was applied to 57 MER-IS images from 2003 to 2009 which were pre-processed through geometric correction,atmospheric correction,etc.At last,a series of maps about Chlorophyll-a concentration distribution of Lake Dianchi were produced.The result showed that Chlorophyll-a con-centration was rising slowly with a periodic fluctuation in Lake Dianchi.During a year,Chlorophyll-a concentration showed a de-creasing-increasing-decreasing pattern,where the minimum appeared in February and March,and the maximum occurred within September and November.Regarding to spatial distribution,Chlorophyll-a concentration was higher at the edge and the north of Lake Dianchi,which are close to the Kunming city and small towns.In addition,the higher Chlorophyll-a concentration was,the more obvious the spatial variation was.