A multiphase opaque glaze with opalescent color was prepared in a basic composition glaze system with some phase separation agent. Effects of the B2O3/P2O5 ratio on the opacifying power and microstructure of the glaze, and opalescent mechanism were studied with scanning electron microscope (SEM), X-ray diffraction (XRD) and colorimeter. The results indicate that the droplets of the liquid phase separation and the crystals precipitated out in the glaze enhance the refraction and the scattering to light, resulting in glaze opacifying. With the mol ratio of B2O3/P2O5 as 1:2 in the selected composition and fired at 1200oC, the glaze possessed the highest whiteness. It is testified that a function transformation existed in glaze, which results in a multiphase coexistence, together with the size, content, contribution of the droplets and crystals have great influence not only on the opacity and the whiteness, but also on the bright and jade likeness of the glaze. The mathematical relation of opacity intensity and the characteristics of the particles in the glaze were also established based on the related analysis.