Abstract: There has been long controversy over the formation age of the Xiaomiao Formation in the eastern part of East Kunlun oro-genic belt due to the lack of precise age. Choosing the meta-detrital rocks of Xiaomiao Formation in Tatuo area in eastern East Kun-lun orogenic belt as the study objects, the authors used LA-ICP-MS zircon U-Pb isotope geochronologic method to study the for-mation age of Xiaomiao Formation. The result shows that it has a wide age range (1551~2624Ma) which can be evidently divided in-to four groups, namely 2624~2603Ma, 2501~2290Ma, 2196~2036Ma and 1921~1551Ma, and the last group can be further divided into two subgroups of 1921~1690Ma and 1641~1551Ma. The age range 1921~1690Ma with the most intense peak value of 1783Ma represents a metamorphism and magmatism of the source region whereas the age range 2501~2290Ma represents a metamorphism of the source region. The detrital zircon also has the age range 2196~2036Ma with some detrital zircons being 2600Ma, which implies that there exists a Neoarchean basement in East Kunlun. The age range 1641~1551Ma (weighted average age equal to 1597Ma) repre-sents the metamorphism time of Xiaomiao Formation, and the latest metamorphism age of the source region is 1690Ma, which re-stricts the formation age of Xiaomiao Formation, which should belong to Mesoproterozoic. The contrast of the metasedimentary zir-con age range and metamorphosed basement tectonic-thermal event of the East Kunlun Mountains shows that the main sediments of Xiaomiao Formation were from Mesoproterozoic Baishahe Formation, and East Kunlun area and North China Craton have similar tectonic evolution processes from the end of Paleoproterozoic to the beginning of Mesoproterozoic, as shown by contrasting their tec-tonic-thermal events.