Abstract: Abstract: During 1∶250000 regional geological survey of the Aksay(Aksayqin) Lake Sheet, the authors found several small biotite monzogranite bodies in the Bayan Har basin at the south slope of the West Kunlun Mountains on the northwestern margin of the Qinghai-Tibet Plateau. Geochemical research on one larger rock body of them indicates that it consists of metaluminous A2 (PA)-type granite, enriched in LREE and LILE and depleted in HREE and HFS, with a negative δEu anomaly but no depletion of Nd. TIMS single-zircon U-Pb dating gave an age of 10.0±0.1 Ma (MSWD＝0.66) for the rock, indicating that the rock is the product of late Miocene magmatism. It may have formed by lithospheric delamination in the late Miocene post-orogenic stage and magma was emplaced in an extensional zone of the gigantic Altyn Tagh strike-slip fault system, which was the manifestation of the stage of strong activity of the Altyn Tagh strike-slip fault system. This discovery has great significance for the study of the tectonic evolution and uplift of Qinghai-Tibet Plateau in this area.