Monazite U-Th-Pb dating plays an important role in isotopic geochronology study. Identification of the genesis and interpretation of the age for monazites are the points. The internal texture, mineral assemblage, REE and trace element features are compared in detail in the paper to determine the identification of magmatic, hydrothermal and sedimentary monazites. Study case of monazite U-Th-Pb geochronology on the tectonic evolution of Sanjiang Tethys gets further discussed. Monazites formed in different geological environments generally display distinguished internal textures, mineral assemblages and elemental compositions: though the zonings developed in monazites are not as perfect as those in zircons, magmatic monazites having paragenesis with brabantite show typical oscillatory zoning possibly which are broad and straight, as well as are characterized by high trace elements (Y, Sc, Th, U) content, Th/U ratio which has positive correlation with that in zircon, enrichment in heavy rare earth elements and Sm, Gd, relatively depleted in Sigma REE and La and Ce, and display intensely depleted rightward REE distribution pattern. Hydrothermal monazites having paragenesis with huttonite display sector zoning probably which reflect the activity of fluid, sedimentary monazites with irregular morphology usually are associated with authigenic mineral. In addition, they are both distinguished from magmatic monazites by their low trace elements content, Th/U ratio which has no stable correlation with that in zircon, enrichment in Sigma REE and La, Ce, relatively poor in light rare earth elements and Sm, and Gd, and display typical tetrad effect REE distribution pattern. The monazites U-Th-Ph ages constrain the earlier Barrovian metamorphism in the interval ca. 204 similar to 190Ma and the local thermal perturbation at ca. 168 similar to 158Ma throughout the Songpan-Ganzi Orogenic Belt, the emplacement of Gangdese leucogranite as well as the crust shortening and thickening with tectonic uplifting at ca. 140 similar to 130Ma, magmatism and left-lateral strike-slip movements in relation to extension occurring coevally from 33Ma to 22Ma along the Red River shear belt (unroofing in the north took place almost 9 million years earlier than that in the central gneiss segments of the shear), and the extrusion structure within the eastern Himalayan syntaxis at ca. 8 similar to 11Myr.