The combined effect of desulfurization and ash removal in the process of coal desulfurization by microwave with peroxyacetic acid was determined by evaluating variations in minerals and sulfur forms using X-ray diffraction and X-ray photoelectron spectroscopy. Dielectric properties of minerals or organic sulfur functional groups in coal were studied utilizing a vector network analyzer in the 100 MHz-6.5 GHz frequency range. Results showed that the trend of ash reduction was similar to that of desulfurization with the treatment time. The main minerals removed were kaolin, montmorillonite, and pyrite under microwave with peroxyacetic acid. The influences of minerals on the dielectric properties of pure coal were higher than those of organic sulfur containing groups. Peak values of the complex permittivity imaginary part (epsilon '') and magnetic loss tangent (tan 5) for pyrite or montmorillonite were significantly higher than those of the other model materials. In addition, it is found that the dielectric property curves of the mixed samples contained the characteristic peaks of the corresponding minerals or organic sulfur compounds. Thus, the sulfur-containing substances of coal could be selectively removed with heating selectivity under the microwave of a particular frequency. These findings confirmed that the cause of the combined effect was the difference of dielectric properties between minerals and organic sulfur-containing groups.