Abstract: This paper presents the distinct characteristics of biaxial crystals caused by inherent stress using cone-shape d interference in the samples. The results of optical experiment show that the characteristics of light uniformity depend greatly on the concentrations of quaternary ammonium salt in solution. Supernormal transmittance ratio properties are reported of KDP crystals doped with 0.001% quaternary ammonium salt. These pheno mena show that the wave-plane effect and the optical activity are highly displayed in KDP crystals, and the effect varies with the three-dimensional positions.