This paper is devoted to the study of muscovites, the most extensively distributed rock-forming minerals in the Lancang metamorphic belt of western Yunnan. Their composition, polytypes and bo values are analysed and examined in detail, and theca regularity of their growth is tentatively discussed. It is shown that the overwhelming majority of muscovites are phengites of 3T+2M1 type whereas those associated with glaucophane are mainly 3T phengites. Studies also indicate that the Lancang metamorphic belt underwent high-pressure metamorphism of blue schist subfacies and was closely related to the evolution of paleotethys. Therefore, it can be correlated9 with other well-known high- pressure belts i n the world.