Abstract: A type of in-phase stepped square radiator in bending vibration is studied in this work. Taking a flat square steel plate as an example, side length and thickness are 50 mm and 6 mm. The frequency of fourth order intrinsic mode is 19332 Hz. The flat square plate driven by a longitudinal vibration transducer with the same resonant frequency at its center is able to create a bending vibration mode, whose four nodal lines locate at 45ångle with each side(there's no such mode shape in its intrinsic eigenvibration). In order to get a highly directional sound, the surrounded areas of the four isosceles triangles formed by each nodal line and each right triangle are modified to be raised ones, the flat square plate radiator is modified a stepped radiator, so that the surface elements vibrating in phase and phase cancellation can be avoided. The calculated radiation pattern of the stepped radiator is better than that of the flat one, whose configuration is similar to that of the theoretical piston one. The measured vibrational mode shape of the stepped radiator is in good agreement with the calculated one. The result of further calculation shows the frequency of itself is lower and its directivity is still shaper by comparison of the square plate stepped radiator and the same linear dimension of rectangular or circular stepped radiator, so it is expected to be used in ultrasound distance measurement and level measurement in stock.