Abstract: Gold nanoparticles in size of 10 nm，30 nm and 50 nm were prepared by improved trisodium citrate reduction procedure. In media of EDTA-NaOH，nanogold exhibited strong catalytic effect on the slow particle reaction of Cu(II) and N₂H₄HCl. The product of copper particles exhibited an absorption peak at 750 nm. When the concentration of nanogold increased, the absorption value at 750 nm enhanced linearly. For nanogold in size of 10, 30 and 50 nm, their linear range, regress equation, detection limit are 0.12~1.68, 0.36~2.80, 1.00~5.00 nmol/L; ΔA_{750 nm}=0.3205C_Au+ 0.0076, ΔA_{750 nm}=0.2201C_Au+0.0056,ΔA_{750 nm}=0.1150C_Au+0.0066;0.05, 0.20, 0.50 nmol/L Au, respectively. The 0.50 and 1.00 nmol/L nanogold in size of 10 nm were determined by 10 times, their relative standard deviation was 4.2% and 3.5% respectively.