Abstract: Allelochemical, which is produced and released by some allelopathic species, is the media that constrains or promotes the coexisting species.Phytoplankton allelopathy has been paid more and more attention, and more detailed studies about the phenomenon of the allelopathy were observed and described, however the property and structure about most allolochemicals still remain unknown.In this overview, the defination, classification, action mechanism of allelochemicals were summarized which based on several kinds of allelochemicals that have been found at present.The allelochemicals found mainly included esters, phenolic acids and terpenoids.Moreover, the research progress about extraction, separation and identification of allelochemicals were introduced emphasisly, the insufficient and some new ideas were also pointed out.Establishing systematic method to obtain allelochemicals and achieving to analyze them in marine areas will be the final objective.