Mucilage sheath is critical for maintaining predominance of colonial Microcystis in eutrophic freshwaters. To explore mechanism underlying the decline of Microcystis bloom, colony size, polysaccharide content and sheath structure of Microcystis colonies were investigated from October to January when the bloom decline. The results indicated that the proportion of Microcystis with colony size larger than 180 μm decreased, while colonies in the 100-180 μm size range increased. Polysaccharide content per cell decreased during the decline period. The scanning electron micrograph showed that sheath structure of Microcystis colonies changed from complete to broken. Element analysis of the surface of Microcystis colonies showed the percentages of Na and P decreased gradually, Al and S fluctuated, while Ca and Si increased. These results indicated that changes in polysaccharide content and element composition of colony sheath accompanied with colonial Microcystis disaggregation, and thus led to bloom decline.