Eutrophication tends to stimulate rapid explosive growth of cyanobacteria, leading to water bloom. Some cyanobacteria can release neurotoxins, hepatoxins, dermatoltoxins, gastrointestinal toxins and cytotoxins, which can be toxic to humans and aquatic animals. How to control the undesired cyanobacteria growth in an aquatic ecosystem? It has been reported that allelopathy and allelochemicals of aquatic macrophytes have certain effects in dealing with this tough issue. It is also reported that vetiver, an emergent plant with flourishing roots, can not only absorb water bodies nitrogen and phosphorus but also secret some allelochemicals to inhibit the growth of cyanobacteria. In the planned experiment on some powerful allelochemicals that can control the growth of cyanobacteria or other floating algae effectively, the inhibitory effects of vetiver oil on Microcystis aeruginosa and Scenedesmus obliquus, when the two algae species were cultured alone or cultured together, were respectively found and discussed for the first time in this study, and the mathematical models were also established about the inhibition of vetiver oil on those two algae species in mixed culture. Moreover, the composition of vetiver oil was assayed and identified by GC-MS. The results showed that vetiver oil had excellent inhibition on M. aeruginosa and S. obliquus in both pure and mixed culture. The EC₅₀ of vetiver oil on these two algae species in pure culture were 0.20 and 0.30 mL/L respectively, which indicated that the inhibitory effects of vetiver oil, like most other aquatic macrophyte and allelochemicals, has species specificity, i.e., the inhibition of vetiver oil on cyanobacteria (M. aeruginosa) is much better than that on green algae (S. obliquus). Because cyanobacterial dominance is most pronounced during the summer months, there is the huge potential for vetiver oil to develop an effective biological cyanobacterial inhibiting agent. The allelochemicals in vetiver oil assayed and identified by GC-MS showed that the main chemical components of vetiver oil are sesquiterpenes, including 1-cyclohexyl-butanone, terpineol, cedar alcohol, cuparene, α-chamigrene, δ-cadinene, α-cadinene and -cedrene in addition to some alkanol and ketone, and among them sesquiterpenes have the most powerful allelopathy according to the papers reported by far. The mathematic models of mixed culture established through the analysis of the results of the experiment could provide the species densities of M. aeruginosa and S. obliquus at any time point as well as that of the concentration of vetiver oil for the best inhibition. Because the mathematic model related to allelopathy studied relatively very limited, the mathematical model undoubtedly has some reference application value for using allelochemicals to inhibit the growth of algae, especially for two algae species in mixed culture. All in all, this research has important theoretical and application value on prevention and control of cyanobacteria blooms, as well as comprehensive utilization of vetiver oil.