Abstract: Radial mode resonator of a piezoelectric disk is extensively applied to piezoelectric measurement; It has been a major subject of piezoelectric measurement in international standardization. Mason researched this mode at first, and he derived the electrical admittance formula and approximate equivalent circuit that is only suitable to the vicinity of the fundmental frequency. In this article, based on the admittance formula derived by Mason and using the Mittag-Leffler's theorem, the complete equivalent circuit have been derived that is suitable to any frequency. It is proved that the complete equivalent circuit corresponds to the strict analytic solution derived from wave equation.