Based on monthly investigation of diatom community and the related environmental conditions during six years in Shahe Reservoir, Lake Tianmu, Liyang City, and investigation of the bloom seasons in Hengshan Reservoir, Yixing City, the environmental and biological characteristics of diatom bloom in reservoirs of Taihu Basin has been analyzed. It was shown that the diatom bloom season in these reservoirs normally happened from May to July. The diatom biomass started to increase significantly when air temperature was above 16℃, and reached the highest growth rate when air temperature was 26℃. The predominant genera of diatom in these reservoirs were Synedra, Achnanthes, Cyclotella and Melocira. Synedra was the dominant species in the diatom community. Being meso-trophic condition in most of these reservoirs, both climate condition and high nutrient level were important to cause diatom blooms. What total nitrogen is lower than 1.0 mg/L or total phosphorus is lower than 0.025 mg/L can create the limitation effect on the diatom biomass in these reservoirs. When the nutrient concentration is higher than those levels, climate condition will be significantly control to make the severity of diatom bloom in the reservoirs.