Severe outbreaks of cyanobacteria cause serious problems with regard to effective utilization of water-resources. As a means of controlling undesired cyanobacterial growth, reed that lives worldwide could be used. Although a feasibility of controlling cyanobacterial growth using reed exists, the causes of growth inhibition and how the effect of inhibition change are still unknown. In this experiment, extraction solution from reed (ESR) sampled every each season were used in laboratory to confirm allelopathic effects of reed on cyanobacterial growth and were analysed in GC/MS to identify anti-cyanobatcterial allelochemicals. The result showed that the cyanobacterial (Phormidium tenue) growth was significantly inhibited by extracted solution. The correlation between the level of inhibition for P. tenue's growth and the DOC concentration was observed. The seasonal changes of DOC concentration in extracted solution from reed was confirmed, too. It was suggested that fatty acids such as Pelargon acid and Myristic acid in extracted solution from reed contribute to allelopathic effects on the growth of P. tenue probably.