A detailed survey of 40 Ar/ 39 Ar dating was carried out on basement rocks of the Eastern Kunlun Mountains. The samples were collected from Jinshuikou, Kuhai, Xiaomiao, Wanbaogou and Nachitai groups. All the samples were analysed in 40 Ar/ 39 Ar isotopic dating laboratory of Salzburg University in Austria. The results of 40 Ar/ 39 Ar dating include: (1) The Jinshuikou Group shows hornblende age of 388 5Ma, muscovite at 233～227Ma, and biotite ages between 232 2 and 208 2Ma. These are interpreted to result from amphibolite|grade Caledonian orogenic diastrophism and low|grade metamorphic Indosinian overprint. (2) The Xiaomiao Group is characterized by a 40 Ar/ 39 Ar muscovite age of 413 8Ma. (3) The Wanbaogou Group gives out a muscovite age of ca. 160Ma to the S of the Central Kunlun fault. (4) The Nachitai Group revealed a biotite age of ca. 110Ma that was overprinted by a very|low|grade event at 60～40 Ma. (5) The Kuhai basement to the S of the Central Kunlun fault is again characterized by a Caledonian age without detectable late overprint: Hornblende: ca. 405Ma, muscovite 376～357Ma, biotite: ca. 360Ma. The new ages constrain that the Kunlun basement essentially formed during Caledonian tectonic events. The basement was locally overprinted by Indosinian tecto|thermal event in the north of the Central Kunlun fault, and by Jurassic and Paleogene tecto|thermal events in the south of the Central Kunlun fault. A detailed survey of 40 Ar / 39 Ar dating was carried out on basement rocks of the Eastern Kunlun Mountains. The samples were collected from Jinshuikou, Kuhai, Xiaomiao, Wanbaogou and Nachitai groups. All the samples were analyzed in 40 Ar / 39 Ar isotopic dating laboratory of Salzburg University in Austria. The results of 40 Ar / 39 Ar dating include: (1) The Jinshuikou Group shows hornblende age of 388 5Ma, muscovite at 233 ~ 227Ma, and bioties ages 232 2 and 208 2Ma. These are derived from amphibolite | grade Caledonian orogenic diastrophism and low | grade metamorphic Indosinian overprint. (2) The Xiaomiao Group is characterized by a 40 Ar / 39 Ar muscovite age of 413 8Ma. (3) The Wanbaogou Group gives out of a muscovite age of ca. 160Ma to the S of the Central Kunlun fault. (4) The Nachitai Group revealed a biotite age of ca. 110Ma that was overprinted by a very | low | grade event at 60-40 Ma (5) The Kuhai basement to the S of the Central Kunlun fault is again characterized by a Caledonian age without detectable late overprint: Hornblende: ca. 405Ma, muscovite 376-357Ma, biotite: ca. 360Ma. The new ages constrain that the Kunlun basement preferably formed during Caledonian tectonic events. The basement was locally overprinted by Indosinian tecto | thermal event in the north of the Central Kunlun fault, and by Jurassic and Paleogene tecto | thermal events in the south of the Central Kunlun fault .