Abstract: For determination of trace amount of mercury in waste water, about 25 mL of the water sample was taken and reacted with 0.40 g of a mixture of SnSO₄ and NH₂SO₃H (mass ratio 3:1) for 15 min to reduce Hg(Ⅱ) to Hg(0) in its vapor state, which was carried over at a flow rate of 0.3 L·min^-1, into an absorption solution, containing 0.20 g of a mixture of KIO₃ and NH₄HSO₄ (mass ratio 1:4), about 5 mL of deionized water and 0.100 mL of 1 mol·L^-1 H₂SO₄ solution, to oxidize Hg(0) to Hg(Ⅱ). 0.10 g of a detection reagent mixture, which contained cadion as chromogenic reagent for Hg(Ⅱ), was then added to the absorption solution, to react with Hg(Ⅱ) to form a colored chelate, and its absorbance was measured after reaction for 2 min. The components in the detection reagent mixture were given as follows:(a) buffer mixture of NaOH and Na₂B₄O₇ (equivalent to 0.1 mL of 2 mol·L^-1 NaOH solution and 0.04 g of Na₂B₄O₇), (b) Triton X-100 (equivalent to 30 μL of 10% volume fraction of Triton X-100 solution), and (c) cadion (equivalent to 60 μL of 1 g·L^-1 alcoholic solution of cadion). The above mentioned procedure was carried out with a portable specialized apparatus and one determination could be completed within 20 min. Detection limit (3s/k) found for this method was 2 μg·L^-1. Test for precision was made at the concentration levels of 0.050, 0.500 mg·L^-1 of Hg, giving values of RSDs (n=6) of 3.6% and 5.1% respectively. Test for recovery was made by standard addition method using waste water sample as matrix giving values of recovery in the range of 92.0%-103%.