Abstract: The highly sensitive colour system of Selenium (Ⅳ)-Potassium iodide-Malachite green was studied. The optimum conditions of this system using spectrophotometry were determined, and the reaction mechanism was discussed. It is feasible to use this method at room temperature. Both of the sensetivity and reproducibility are satisfactory. The molar absorptivity is 1.2 × 10⁵ 1 · mol^-1· cm^-1.Beer's law is obeyed in the range of 0~0.32μg/ml. It has been successfully used in determinining the trace selenium presented in water.