Abstract: Uni-culture and co-culture experiments on Microcystis aeruginosa and Chlorella vulgaris were carried out at pH=5,6,7,8,9,10 and 11 under controlled laboratory conditions.The effects of pH on growth and competitiveness of these two algal species were studied.The growth and competing characteristics of M.aeruginosa and C.vulgaris were analyzed using the logistic equation and the Lotka-Volterra model. The results showed that the effects of pH on growth and competitiveness of the two algae were significant; M.aeruginosa grew better when pH=8-11,with an optimal pH of 9; and C.vulgaris grew quickly when pH=7-9,with an optimal pH of 8. In the co-culture experiments,M.aeruginosa had a competitive edge over C.vulgaris,with a β value higher than α at pH=8,9 or 10. However,at pH=7,C.vulgaris gained the upper-hand,with the α value higher than β.