A measurement method of the dielectric constant of materials whose standard sample is not prepared easily is proposed. The unprepared sample of the material is placed in a cavity, and the resonant frequency is measured. A commercial software simulates the same sample and the cavity leading to find the correct dielectric constant. The measured samples include a ceramic, a forced glass, and a powdered enamel. The measured dielectric constant of a ceramic, a forced glass, and a powdered enamel are 11-j0.0033(${epsilon}_{gamma}=11,;tan{delta}=0.0003$), 4.15-j0.053(${epsilon}_{gamma}=4.15,;tan{delta}=0.0128$), and 3.9-j0.042(${epsilon}_{gamma}=3.9,;tan{delta}=0.0108$) respectively.