The measurement of cell death constant in Anabaena flos-aquae was tested by the Live/Dead BacLight Viability kit(Molecular Probes Co., Seatle, WA). When the Live/Dead BacLight Viability kit was applied to Anabaena flos-aquae, the cells with intact cell membranes(live cells) stained fluorescent green, while the cell with damaged membranes(dead cells) stained fluorescent red and the background remained virtually nonfluorescent. The rations of live : dead cells in the cell suspension were controlled artifically and Live/Dead BacLight Viability kit was applied to them. The ratios of green:red fluorescent cells in the cell suspension were the same as those of live : dead cells controlled artifically. It was also approved by the fluorescence emission. The cell death constant was measured in the P-limited Anabaena flos-aquae chemostal culture in the N-fixing and $KNO_3-supplied$ conditions. The culture in N-fixing chemostat had a dead cell proportion of 1.2% at the growth rate of 0.7/day and increased to 2.6% at the growth rate of 0.3/day. The cell death constant of N-fixing culture was 0.008/day.There was a same trend in the $KNO_3-supplied$ chemostat culture. The proportion of dead cell was 1.6% of dead cell proportion at the growth rate of 0.7/day and increased to 4.3% at the growth rate of 0.3/day.