An ultrasonic treatment instrument includes a piezoelectric element assembly, a probe, a backing plate, an electrode assembly, a cylinder and a cylindrical inner cover. The piezoelectric element assembly includes piezoelectric elements and generates ultrasonic vibration. The probe is connected to the piezoelectric element assembly and transmits the ultrasonic vibration. The backing plate is connected to the piezoelectric element assembly. The electrode assembly includes electrodes respectively clamped between the piezoelectric elements, electrifies the piezoelectric elements and generates the ultrasonic vibration. The cylinder is connected to the probe and locates the piezoelectric element assembly, the backing plate and the electrode assembly therein. The cylindrical inner cover includes an inner diameter larger than the backing plate and an outer diameter smaller than the cylinder. The inner cover is disposed between the backing plate and the electrode assembly, and the electrode assembly is disposed between the cylinder and the inner cover.