Abstract: Copper(Ⅱ) can effectively inhibit the photo-induced reaction of I₂formation. Since there exists a quantitative relationship between irradiated time to the end-point versus concentration of copper(Ⅱ), therefore copper (Ⅱ) contained in samples can be quantitatively determined.The optimal experimental conditions have been examined. The interference behaviour of about 20 foreign ions were observed. This new method for the determination of micro-copper (Ⅱ) has been used in the analysis of tap water, lake water and cabbage samples and satisfactory result was obtained.The end-point was determined by "die-stop titration" method. The data were processed with a micro-computer,using a program written for lagrange's interpolation formula fitting to the nonlinear calibration curve.