Abstract: An ultra-sonic dog-expelling device, which comprises an upper casing, a lower casing and an ultra-sonic oscillation circuit; the tuner walls of the upper and lower casings are furnished with click grooves and fastening tongues respectively so as to have the two casings assembled together quickly. The inner side of each casing is furnished with a screw hole so as to facilitate the casings to be assembled into a closed case. By means of an intermittent unstable multi-oscillator, an intermittent oscillation frequency can be provided, and can be convened into an ultra-sonic wave through an ultra-sonic wave transducer located in the round hole at the front portion of the two casings to annoy and interfere the hearing sense of a dog so as to expel the dog away and to prevent it from attacking or nearing a person.