Nearly all geothermometer are thermometer of magmatic or metamorphic processes. Rhombic section. angle thermometer can determine the temperature of deformation, so it can be served as a deformation thermometer. Pericline twinning, whose composition plane is rhombic section，is formed mainly ,by mechanical deformation. The position of rhombic section varies with both chemical composition and structural state of plagioclase, which itself is a function of temperature. Consequently the position of rhombic section is depe- ndent on the temperature of deformation, during which Pericline twinning Was formed. In Ekou granite body of Wutaishan area 49 angles of rhombic section and corresponding compositions of plagioclase have been determined and 49 tempe- rature values have been yielded. On histogram (Fig. 4) there are two tempera- tune peaks 520℃ and 480℃, which are similar to 494-508℃ inferxed from garnet-biotite pair of country rocks. It means that temperature of deformation is in agreement with temperature of greenschist metamorphism. In Wutaishan area durimg greenschist grade metamorphism two epizodes of deformation have been olisaived, which -coincides with the two temperature peaks: Therefore rho- mbic. erection angle thermometer provides , a means for study the relation, petyy- een deforrciation and metamorphism.