The field dependence of electric birefringence for calf thymus DNA nad λ DNA at various pH and various ionic strength was measured. The monodisperse and rigid polyions show linear dependence of electric birefringence on the electric field. In order to explain the experimental results the induced moment of a linear polyion is calculated based on the model that a polyion molecule is represented as a line of uniform charge density and the counterions are represented as oppositely charged cloud in a narrow and deep potential valley around a polyion molecule. The orientation factor and the electric birefringence are also calculated. According to the results of these calculations, for large polyions the induced moment is constant, that is, independent of the applied field except in very low electric fields. When the induced moment is constant the orientation factor or the electric birefringence shows linear dependence on the field strength. However, small polyions show quadratic dependence on the field strength. Large polyions usually show linear dependence on the field strength, but they often also show quadratic dependence in very low electric field. The experimental results of electric birefringence for DNA agree with the ones predicted by the theory.