Abstract: The method for direct titration of metal ions with EDTAwas investigated. The ligand-exchange reaction Ni-Trien+Cu-EDTA→Cu-Trien+Ni-EDTA was used as indicator reaction, which can be catalyzed by trace amount of EDTA. Cu-Trien was measured spectrophotometrially at 550nm,and used as indicator substance, then the change of the reaction rate of the indicator reaction was measured. By this method, the micromole level of Cu,Ni,Zn,Pb and Ca can be determined The titration was carried out in a borax buffer solution with. pH 10.0.For Cu²⁺, Zn²⁺,Pb²⁺, and Ni²⁺, because of the conditional stability constants of their EDTAcomplexes being greater than the corresponding value of Cu-EDTA, they can catalyze the indicator reaction, therefore the titrable range is narrow. But for the metal ions such as Ca²⁺, whose EDTA complex's conditional stability constant is less than the corresponding value of Cu-EDTA, the titrable range is wide, because it is of no catalytic effect on the indicator reaction.