A simple and rapid spectrophotometric method for the determination of oxalate ion was established by the fading of a colored complex between N,N′-diethyl-N,N′-[[4,4′-dihydroxy-1,1′-binaphthalene]-3,3′-diyl]bisbenzamide and copper(II). Beer's law was obeyed in the concentration range of 0.1 - 2.0 µg cm^-3 for oxalate ion, with an effective molar absorptivity at 533 nm and the relative standard deviation being 8.0 × 10³ dm³ mol^-1 cm^-1 and 1.0% (n = 5), respectively. This proposed method has excellent reproducibility, and was applied to recovery tests of oxalate ion in tap water and human urine; the results were satisfactory. This is suggested that the method is based on the reaction of copper(II) to copper(I) with oxalate ion.