In this paper, the authors used such means as microscope, XRD and EPMA to study the origin and distribution characteristics of iridescent in labradorite. The results indicate that the inner lamellar structure of labradorite causes interference effects which contribute to the iridescent colors. Furthermore, each unit lamellar structure is made up of calc-poor and calc-rich plagioclase feldspar. Only when the thickness of the lamellar structure varies between 128 nm and 292 nm can the interference colors be visible. Because the surface of the lamella is not absolutely smooth, iridescence always looks like the contour line of a map. Additionally, the thickness of the lamella is related to calcic content, i.e., the more the calcic content and the thicker the lamellar, the longer the wavelength of color. The uniform interference color may have different calcic contents, varying from place to place and from environment to environment. Nevertheless, it is demonstrated that the relationship between the iridescence and the calcic content of labradorite is positive at the same place.