Abstract: An inhibitory effect of Acorus tatarinowii on algae growth was found. In addition to the competitions of light and mineral nutrients between Acorus tatrinowii and algae ,the mechanism of this inhibitory effect was mainly due to an excretion of some organic substances from the root system and rhizome of Acorus tatarinowii which may injure and abate algal cells(allelopathic effect). When algal cells were treated with cultured water of Acorus tatarinowii, chlorophylla was destroyed, the photosynthetic rate of algae markedly declined and the ability of cells to reduce TTC decreased. Under fluorescence microscope, it was seen that algal cells turned from bright red to bluish green.