The rate constant of Nucleophilic addition of sodium thiophenoxide to nitrone were determined by UV Spectrophotometry and a rate equation which can be applied over wide pH range was obtained. Base on the rate equation, general base effect, substituent effect and final product, plausible mechanism of addition reaction have been proposed. Below pH 3.0, the reaction was initiated of thiophenol, and in the range of pH 3.0~10.0, proceeded by the competitive addition of thiophenol and thiophenoxide anion. Above the pH 10.0, the reaction proceeded through the addition of a thiophenoxide anion.