To elucidate how the environmental factors affect water-bloom of Microcystis aeruginosa population, abundance of phytoplankton and nine environmental factors were measured in a small, eutrophic reservoir from March to August, 1993. The data were transformed into stationary data by ARIMA model and the cross correlation coefficients among time series were calculated. Anoxic zone developed after March while thermal stratification did after April. Soluble reactive phosphorus(SRP) concentration was drastically increased since May and M. aeruginosa appeared in June and dominated in July. As the result of cross correlation analysis, abundance of M. aeruginosa lagged behind the increase of SRP concentration which coincided with the development of anoxic zone, which in turn lagged behind the increase of bottom water temperature. Ammonium ion concentration also led to abundance of M. aeruginosa. Moreover, abundance of M. aeruginosa coincided with water column stability which lagged behind surface water temperature. Increase of water temperature would develop anoxic condition and tncrease water column stability. Under anoxic condition, SRP and NH_4^+ would be released from sediment and facilitate growth of M. aeruginosa population. Besides, high water column stability would facilitate buoyancy of this species.