The generation, due to glide motion of dislocations, of the second harmonic of an ultrasonic stress or strain wave propagating in a crystal is studied by using the string model of dislocations in the presence of a static bias stress. The calculated results agree semiquantitatively with the experiment by Hikata, Chick, and Elbaum. The dependence of the amplitude of the second harmonic on bias stress, when this stress is sufficient to cause dislocation unpinning, can be accounted for by the dislocation damping effect that is associated with the anomalous velocity change due to unpinning.