The present study aimed to evaluate the contribution of stress factors to the decline of Microcystis bloom, also tried to develop some indicator of decline. Four stress conditions including nitrogen starvation, phophorus starvation, dark and low temperature (10 degrees C) were designed, Decrease of biomass and the concomitant change of antioxidant system, including superoxide dismutase (SOD) and catalase (CAT) activities were detected. The result indicated that growth was significantly inhibited under dark and low temperature. Microcystis population declined after 25 days tinder dark, while declined after only 7 days under low temperature. Compared with the physical stress, nutrition such as nitrogen and phosphorus did not limit the growth effectively in the early stage. Chlorophyll-a decreased as soon as being incubated into nitrogen free BG-11 medium. After being kept in phosphorus free medium for 20 days. chlorophyll-a reached peak, about the twice of the initial concentration, and then decreased at a rate of 0.1 mg/(L.d), which was the highest decaying rate among the four factors. It indicated its significance of available phosphorus for Microcystis survival and decline. According to the results, when phosphorus was limited, or water temperature was decreased to 10 degrees C, cautions should be taken into account for the decrease of dissolved oxygen and possible increase of toxins or odors substances from the declining bloom, In healthy Microcystis, SOD activity was about 0.06U/mg protein, while CAT activity was too low to be measured. In addition, SOD activity increased earlier than CAT activity when Microcystis was exposed to stress conditions. CAT activity increased only when the content of chlorophyll-a began to decline. Both of the increase of SOD and CAT activities correlated negatively with the decrease of the content of chlorophyll-a though there were some exceptions. Combination of the two parameters would be potential indicator of bloom decline.