new kinetic spectrophotometic method for the determination of trace amount mercury(II) in Tap water and River water samples has been described based on it s the catalytic effect on the oxidations of barium die phenyl aminosulfunate, by potassium chlorate in sulfuric acid. The reaction is followed spec- trophotometrically by measuring the decrease in the absorbance at 469 nm. Under the optimum conditions of 0.2 molL-1 sulfuric acid, 1.0 × 10-4 mol L-1, barium die phenyl aminosulfunate, 0.01 mol L-1 potassium chlorate at 35oC, calibration graph in the rang of 0.001-0.06 ng L-1 mercury(II). Concentration was obtained with detection limit of 0.2303 g mL-1 by the fixed-time method of 4.5 min. The relative standard deviation for 0.05 ng L-1 mercury(II) is %90. No serious interference was identified. The applicability of the method was demonstrated by the determination of the mercury (II) in Tap water and River water samples.