Abstract: Comparative studies were conducted on algaecidal properties of inorganic,organic and chelated copper-preparations by growth inhibition test of the target algae,Microcystis aeruginosa(the dominant algae of Cyanobacteria algal bloom),and non-target algae,Chlorella vulgaris and Scenedesmus obliquus,and a subsequent 15-d extension of growth inhibition test on Microcystis aeruginosa.Results show that for Microcystis aeruginosa with a initial concentration of 2×10⁵-4×10⁵ mL^-1,the 96-h-EC₅₀ of cupric-amminium complexion,copper(succinate+glutarate+adipate) and copper acetate was 0.03,0.06 and 0.05 mg·L^-1,respectively,which did not change much when the initial concentration of algal cells was increased to 2×10⁶-4×10⁶ mL-1.It was found that 0.25 mg·L^-1 cupric-amminium complexion(aqueous solutions,25%),0.30 mg·L^-1 copper(succinate+glutarate+adipate)(W.P.,30%) or 0.20 mg·L^-1 copper acetate(W.P.,20%) is adequate to completely prohibit the growth of Microcystis aeruginosa,which will not recover and reproduce within 15 days after the application.Besides,being lower in 96-h-EC₅₀ to Microcystis aeruginosa than to Chlorella vulgaris and Scenedesmus obliquus,these copper-algaecides would not pose any significant threat to non-target algae,while inhibiting the growth of Cyanobacteria.It is,therefore,held that cupric-amminium complexion,copper and copper acetate are promising substances that can be developed into highly effective and environmental friendly inhibitors of Cyanobacteria algal bloom.