The LANDSAT Thematic Mappper (TM) data on July 25, 1992 were used together with concurrent measurement of Chlorophyll-a concentration in the northern part of Taihu Lake to relate the TM data quantitatively to Chl-a.Firstly, the statistical relationships between TM data on each band at sample sites and measured Chi-a were analyzed.It is shown that the third band (TM3) data power-law decays with the increase of Chl-a, while the forth band (TM4) and the seventh band (TM7) separately show the positive and negative linear correlations with the Chl-a concentration.These sensitive bands CTM3, TM4 and TM7) were selected as independent variables, whose optimum combinations were examined through their linearly and non-linearly alterations in order to obtain a new quantitative index for Chl-a estimation.Meanwhile, a method of revising this index to remove the influences of ground irradiation changes is presented in this paper.It is found that there is a close linear interrelation between the revised quantitative index and ChJ-a concentration.Based on this interrelation,the final Chl-a estimation model was developed in this paper.The Chl-a values measured at several sample sites in Meiliang Bay on Oct.29, 1992 were coincident with the calculated ones using this model.The statistical analysis and checking results show that this Chl-a estimation model can be applied to images of different dates and areas on similar weather condition because through irradiation amendment the factors of time and latitude have little interference with this model.