[Objectives] To studythe growth inhibition and mechanism by L-Phenylalanine inMicrocystis aeruginosa.[Methods]Algal cells in logarithmic growth phase were inoculated in BG11 Medium andvarious concentrations of L-Phenylalanine (0, 0.625, 1.25, 2.5, 5.0 10.0 and 20.0μg/mL) were added. The cells were cultured at 28°C under a 12L/12D cycle for 24h and the cell density was counted in a haemacytometer chamber by light microscope. Besides, EC50 and 95% Confidence level were calculated. Furthermore, to conducte the time-cource of cell growth inhibition of L-Phenylalanine at concentration of 10.0μg/mL, the algal density wasmonitored every 8h in 24h.The membrane integrity and the esterase activity were detected with flow cytometry after 1h and 24h exposure of 10.0μg/mLL-Phenylalanine.[Results] Thedose-response and time-courserelationships between L-Phenylalanine exposure and the growth inhibition ofM. aeruginosawere observed(r =0.99, p<0.01; r = 0.97，p<0.01). The lowest effective concentration ofL-Phenylalanine to inhibit the growth ofM. aeruginosa was 0.625μg/mL. The higher concentration of 20.0μg/mL L-Phenylalanine could inhibit almost 70% cell growth ofM. aeruginosa in 24h exposure. Consequently, the EC50 value ofL-Phenylalanine for growth inhibition ofM. aeruginosa was6.2μg/mL,95% Confidence level was from 0.005μg/mL to 16.76μg/mL in 24h, respectively. Furthermore, the growth inhibition ofM. aeruginosa took place as early as 8 h exposure to 10.0μg/mLL-Phenylalanine.The membrane integrity of the cells showed significant variations after 24h exposure toL-Phenylalanine. Meanwhile, no effects on esterase activity of the cells were observed after exposure toL-Phenylalanine.[Conclusions] L-Phenylalanine could significantly inhibit the growth ofM. aeruginosa cells, closely related with alterations or disorders in themembrane systems, but not in variation of esterase activity in the cells.