In the handling of microwave heating, it is useful to consider the same type of equations, the heat conduction equation and the diffusion equation. Due to the difference in the diffusion coefficients of each equation, there is a time delay between the energy applied by the microwave and the thermal energy. From this delay, microwave energy is stored non-thermally, which directly breaks the bonds of the crystal (it means phase transition) before it becomes thermal energy. Also, non-thermal energy is treated as acoustic phonon in the crystal. Therefore, we propose to treat phonon as an epi-thermal distribution function with an extended Bose-Einstein distribution, and in fact the application of microwaves forms a monochromatic peak in the distribution function. Its peak decays over time and becomes thermal energy. Such a process is one of the indicators of how to understand the phenomenon of microwave heating.