Based on the electro-optical coefficients, the Miller-δ coefficients of KNbO3, LiNbO3, BaTiO3 and several semiconductor crystals were calculated. The results show that the coefficients are frequency independent and are the same for unclamped and clamped values, so it is feasible to take the Miller-δ coefficients to characterize the nonlinear optical properties. Furthermore, a novel mechanism of the electro-optical effects is presented which maintains that the electro-optical effect results from the self-similarity of the nonlinear system, manifested as the recopy of the linear properties and the Miller-δ coefficients are the proportion factors of self-copy.