Mechanical vibrations of ultrasonic frequency exert a considerable influence upon the mechanical properties of metals, in particular, their deformation characteristics. The following three phenomena are considered: ultrasonic softening, ultrasonic hardening and the influence of preliminary ultrasonic treatment upon the steady-state creep of metals. Ultrasonic softening takes place when a ultrasonic loading is superimposed upon a static one, which results in the decrease of static load needed to deform the metal. Ultrasonic hardening occurs during the sonication of annealed material without a static loading. The ultrasonic hardening manifests itself in the increase of the yield strength of material due to the defects of crystalline structure nucleated and developed in acoustic field. Ultrasonic treatment consists of the ultrasonic irradiation of material and subsequent annealing. Materials treated in such a way can offer a considerable resistance to creep deformation. However, this tendency is not a monotonic function of the preliminary sonification parameters. A new theory, the synthetic theory of irrecoverable deformation, is employed to model the phenomena discussed above.