An open-ended coaxial line probe for dielectric measurements at high temperatures was developed. The probe, which was fabricated using stainless steel as the center and outer conductors and quartz wool as an insulator, can function well up to 800 degrees C. The dielectric properties of the raw materials and resultant materials were measured using a network analyzer that was equipped with the open-ended coaxial line probe at high temperatures to clarify the formation reaction process of YVO4:Eu3+ and Mg2SnO4:Mn2+ phosphors using a microwave heating method. The developed probe was confirmed to be effective for reaction-process analyses at high temperatures.