The euphotic zone is defined as the layer where net phytoplankton production is positive. Therefore, determination of the euphotic depth is useful for estimating primary production. The TM data on 2007-01-07 and 2006-08-01, accompanied by real-time water quality parameters including concentrations of tripton and chlorophyll-a (Chl.a) in Lake Taihu, were used to develop quantitative retrieval models of tripton and Chl.a. Euphotic depth was calculated using the model developed in Lake Taihu based on the concentrations of tripton and Chl.a. The euphotic depth ranged from 0.27 to 2.28m (mean 0.56±0.22m) in winter depth from 0.21 to 2.03m (mean 0.98±0.24m) in the summer. In the winter, the euphotic depths showed spatial distributions which were ordered as: the south of Lake Taihu < the west coast of Lake Taihu < the lake center < Xukou Bay < Gonghu Bay < Meiliang Bay < East Lake Taihu < Zhushan Bay. In summer, the order changed as follows: the west coast of Lake Taihu