Abstract: Inhibitory effect of rice straw (Yanjing 9) on the growth of Microcystis aeruginosa and the inhibitory mechanism were studied. Results show that when 10 g·L^-1 or more of rice straw was added to M. aeruginosa liquid,both sterilized and un-sterilized rice straws could inhibit the growth of M. aeruginosa. In the sterilized group,the respiratory rate and SOD activity evidently increased firstly and then declined rapidly. The result of filter paper tests on solid medium show that among the extractives extracted by different organic solvents (petroleum ether,ethyl acetate and ether) from the leaching liquor of sterilized rice straw,ether extracts could be observed to have inhibitory effects on M. aeruginosa. This indicates that the algae-inhibition effect was caused by some substances in rice straw itself,rather than by microorganisms. Further study show that some organic acids and phenolic compounds in the ether extracts could inhibit the growth of M. aeruginosa. In addition,the smaller the fragments of rice straw were,the more obvious the inhibitory effect was,which might be related to the adsorption effect of rice straw.