Abstract: A stepped circular plate radiator for its big radiating area and advantages of high radiation efficiency has been widely used in high-power ultrasonic field.Because of the frequency equations of a stepped circular plate containing special functions,in addition,picking out the optium solution by applying frequency equations from among the numerous solutions is very time-consuming.Through a lot of calculation concerning the basal flat circular plate,stepped circular plate and the flat plate which its thickness equates to basal thickness plus the one of the convex plate in this paper,we have discovered that the radius and the nodal radius of single stepped plate is very close to the flat plate which its thickness equates to basal thickness plus the one of the convex plate at the same resonant frequency.This will provide a method of determination of the initial parameter value in complex design program of a stepped circular plate.