The spectrum of NO~-_2-KI-Crystal violet ion association complex has the resonance in phosphoric acid medium. Based on this phenomenon,the spectral characteristics, the effect factors and optimum conditions of the reaction have been investigated. The quantitative relationship between the intensity of resonance scattering to the concentration of nitrite ion was established. A new method for the determination of trace nitrite ion in water sample has been proposed. This method can be operated conveniently at ambient temperature. It showed a high s...