Abstract: Pure culture of chlorella was done in solutions different in concentration of Oenanthe javaica extracts added,to explore effects of O.javaica extracts on growth,chlorophyll content and ultrastructure of chlorella.Results show that in the treatment of 10 g·L^-1 O.javaica extract growth of chlorella was significantly promoted,and in the treatment of 20 g·L^-1 O.javaica extract the number of cells and chlorophyll content of chlorella increased with a margin narrower than that in the control in the first 7 days,and afterwards,the growth was inhibited.In the treatments of 30-50 g·L^-1 O.javaica extract,phenomenon of the inhibition became obvious on the fifth day and more obvious with the time going on and with the increasing concentration of the extract added as well.In the treatment of 40 g·L^-1 O.javaica extract,cell walls of the algae broke up and even vanished,chloroplast lamellas in the cells swelled and even disintegrated,and karyolemmas ripped with karyoplasm extravasating.The findings indicate that O.javaica extract had an allelopathic effect on chlorella,and when low in concentration,it promotes growth of the algae,but when high,it acts reversely.