Landsat Thematic Mapper data were processed to estimate the biomass of Cyanophyta in Lake Meiliang, a gulf in the northern part of Lako Taihu. In the summer of 1992, on-the-spot sampling was taken in the gulf on the date of the overpass of Landsat-5 for 16 carefully selected sites to obtain chlorophyll-a and phytoplankton concentrations. With the ground truth data and Thematic Mapper images,two regression models were established between chlorophyll-a and DVI (Differential Vegetation lndex) and between Cyanophyta concentration and DVI. The regression models were the basis of the techniques of mapping the spatial distribution of chlorophyll-a or Cyanophyta concentration and integrating the whole pixels to estimate the total amount of chlorophyll-a and Cyanophyta.It was estimated that the lake contained 2133 kilograms of chlorophyll-a, or 178.2 tones in terms of Chynophyta.