Abstract: Based on chemical method of electron microprobe dating of monazite and previous improved calculating formula of ages and age uncertainties，we put forward a new formula dealing with age error on the basis of general differentiation and wrote a Visual C computer program on the Windows 2000 for calculations of age and age error．Using the new computer program，we recalculated previously published data of known ages and age errors, demonstrating the feasibility of the new formula of age error．The monazites from two metamorphic sedimentary rock samples(KM2127-5，DK2107-2)from the Eastern Tianshan were analyzed for U-Th-Pb compositions．All analyses were carried out using the JXA-8100 electron microprobe．The monazites from sample DK2107-2 record two peaks of metamorphic ages which are 341．0±3．9 Ma and 255．2±3．3 Ma．respectively．The metamorphic age of 341．0±3．9Ma is the most important period，forming an early mineral assemblage of Ky+St+Bt+Pl+Q±Or．The monazites from sample KM2127-5 display one peak of ages．262．3 4±4．4 Ma．Both the 255．2 4±3．3 Ma and 262．3 4±4．4 Ma data represent the second peak period of metamorphism，during which a mineral assemblage of Cord+Bt+P1+Or+Q were produced．The analytical results of monazite dating are well consistent with the previous 40Ar-39Ar ages, indicating that the electron microprobe dating technique for monazite and the new formula estimating age uncertainties are acceptable．