This paper deals with the electromagnetic wave attenuation characteristics by flames and the diagnostics of its temperatures using this characteristics. The temperatures of flames, which small amounts of metals with low ionization potential such as alkali metals are seeded, are estimated from the differences of wave attenuation or phase shifts depending on two different seeding species. This flame temperature estimation method discussed here is similar to the optical one in that a flame temperature is obtained by the mutual intensity comparison of two line spectra. In our experiments, the flames of the propane-oxygen and the solid propellants at one atmospheric pressure are used. The measured attenuation values of mm wave and micro wave agreed approximately with the predicted values from the thermal ionization equation, assuming the flame as the weakly ionized Lorentz plasma, and neglecting the effects of the reflected waves on the flame boundaries. Attenuation correlation between mm wave and micro-wave was good. When the flame used became relatively low loss dielectric media, the wave attenuations were proportional to the phase shifts. It is found from our preliminary experiments that this method is applicable to determine the flame temperatures.