Abstract: To understand the impacts of organic acids released by roots of Pontederia cordata on growth and competition of Microcystis aeruginosa and Scenedesmus obliquus, single culture and mix-culture of the two species were treated with succinic acid at different concentrations in a laboratory control environment. The initial ratio of M. aeruginosa toS. obliquus in BG11 medium was 1∶0 (100% M), 1∶1 (50% M+50% S), 2∶3 (40% M+60% S), 3∶2 (60% M + 40% S) and 0∶1 (100% S). Succinic acid exhibited a strong inhibitory effect on growth of M. aeruginosa and S. obliquus, and the magnitude of such effect was positively correlated with the concentration of succinic acid. In the absence of succinic acid, M. aeruginosa cultured with or without S. obliquushad a faster growth rate than S. obliquus.With the addition of 60 mg·L^-1 succinic acid, β (an inhibitory parameter of S. obliquus toM. aeruginosa) was more than 0 when the ratio of M. aeruginosa to S. obliquus was 2∶3 or 1∶1 and less than 0 when the ratio ofM. aeruginosa toS. obliquus was 3∶2, indicating that the inhibitory effect of one to another varied with the initial proportion of the two species. The succinic acid could change the competition between M. aeruginosa and S. obliquus. Under low concentrations (20, 40 mg·L^-1), M. aeruginosa inhibited S. obliquus, butS. obliquus promoted M. aeruginosa. Under high concentrations (60, 80 mg·L^-1), the performance of both species was mutually inhibited, indicating an intensified competition.