Abstract: Owing to the interference of ordinary and extraordinary rays, the feedthrough effects to and fro between these rays in backscatettering geometry, such as Z(YX)Z, Z(YY)Z etc, are existent. These feedthrough effects will lead to the happening of roundtrip feedthrough and some scattering bands, which apparently violate the group theory selection rules. Calculations based on S. Porto's theory coincide with the experimental results in Raiman spectra of LiNbO₃ and LitaO₃.