Abstract: The volcanic rocks in the Yulinshan Formation to the north of the Bangong Co-Nujiang River suture resulted from alkaline potassic volcanism during the uplift of Qinghai-Tibet Plateau.Since the dis-covery,they have been considered to be late Pliocene to early Pleistocene in age.They may be correlated within the areal extent from southern Qiangtang to Gangdise.According to the isotope chronological study of the volcanic rocks of the Yulinshan Formation,leucite phonolite of the Yulinshan Formation has an in-tergrated 40Ar / 39 Ar age of 27.8Ma and K-Ar ages of 30-18Ma ,which suggests that the age of the Yulinshan Formation should be Oligocene to Miocene.Its distribution is only limited to the area north of the Bangong Co-Nujiang River suture.