Microcystis aeruginosa was co-cultivated with aqueous extract from fresh reaped Oryza sativa L. straw and the inhibiting effect of this aqueous extract on M. aeruginosa was studied in this paper. Results indicated that the growth of M. aeruginosa was obviously inhibited when the rate of quality was over 1/100(rice straw/water). Chlorophyll a content, photosynthetic rate and the intensity of spontaneous fluorescence decreased continuously as the treatment went on ; respiratory rate and SOD activity decreased evidently at the first 3 days of treatment and then increased ; membrane penetrability increased at the first 5 days of treatment and then decreased slowly. Some substances were extracted from the aqueous extract by different organic solvents (ether, ethyl acetate and petroleum e-ther 30-60℃), respectively. Inhibiting effect could be observed with filtering paper methods on solid medium when the substances were extracted by ether, ethyl acetate. This suggested that some inhibitive factors might exist in these substances.