To find a anti-Microcystis microorganism as a biological control agent, we screened all actinomycetes isolated from soil samples of sea beach in Jiangsu Province, China. A strain YC0412 capable of strongly inhibiting the cyanobacteria Microcystis aeruginosa PCC7806 was isolated. Growthes of Anabaena sp. PCC7120, Synechoccocus sp. PCC6803, Chlorella ellipsoidea and Scenedesmus sp. were also inhibited by 32.4%,12.46%, 32.9%, 82.19% and 32.57% respectively with 1/10 volume of YC0412 broth after 3 days . The data showed YC0412 had a wide host rang of antialgal activity. On the basis of morphological, physiological and biochemical characteristics, and 16S rDNA sequence, strain YC0412 was determined to be Streptomyces griseinus. To investigate the property of antialgal substance produced by YC0412, we extracted the culture broth of YC0412 with ethyl acetate or chloroform, and tested the anti-Microcystis activities of different fractions. The ethyl acetate extract fraction resulted in an stronger inhibition of Microcystis. That implied that the active substance should be a low polar and hydrophobic molecular.