Abstract: To search for a new type of algicide,low in toxicity but high in effect,inhibiting effects of β-cycloamylose-chathrated copper(β-CD) chlorophyllin and β-CD ammonium copper chlorophyllin on Synechocystis sp.PCC6803 were investigated.Results show that β-CD copper chlorophyllin and β-CD ammonium copper chlorophyllin were more effective in inhibiting Synechocystis than CuSO4,completely inhibiting growth of the alga when their Cu2+ concentration was 2.5 and 1.5 μmol·L-1,separately.They could decrease the intensity of chlorophyll fluorescence and the activity of nitrate reductase of the alga,but induce the activities of its phosphatase and anti-oxidation system and improve its membrane permeability(P<0.05).It is presumed that the toxic mechanism of the β-CD copper chlorophyllin and β-CD ammonium copper chlorophyllin inhibiting Synechocystis was similar to that of inorganic copper ion.