A nonlinear theoretical treatment is discussed of current saturation observed in a piezoelectric semiconductor such as CdS or ZnO. A phenomenological approach is developed in which nonlinear piezoelectricity, anharmonicity of lattice vibration, nonlinear terms for current density are taken into consideration. A nonlinear differential equation for the potential acting on electrons is obtained and solved by means of mathematical technique in the theory of nonlinear oscillation. The resultant current-voltage relation is shown to have a kink character consistent with experimental results.