Abstract: The interaction between microwave and chemical reaction needs to be further studied for better application of microwaves to chemistry.The effective permittivity can be used to describe the molecular polarization of the mixture in the reaction and consequently be used to calculate the transmission and absorption of microwave in the reaction mixture.An improved formula based on Huang’s empirical formula was proposed to calculate the effective permittivity of chemical reaction at any temperature and reaction time by means of the temperature dependent number of molecules per unit volume of reaction solution.The calculated results of effective permittivity of two different redox reactions were in good agreement with the measured results and better than that of the original formula.