The Asian monsoon-arid environment system began to develop during the Oligocene, but the exact position of the arid zone boundary is uncertain in the Oligocene. Fossil mammal assemblages can be used to assess the environment of an area. There were two sizes of mammals in the Nanpoping fauna from the Oligocene strata of the Lanzhou Basin: the small ones were grassland rodents such as Ctenodactylidae; the large ones were forest dwellers such as Paraceratherium and Paraentelodon. Of the 23 species of mammals identified in the Nanpoping fauna, ~40% were forest dwellers and the rest(~60%) were grassland taxa. The Nanpoping fauna was compared with the contemporaneous Dingdanggou fauna in the Danghe area to the north and Jiaozigou fauna in the Linxia area to the south. The Nanpoping fauna contains elements of both other faunas and thus represents a transitional assemblage between forest and grassland. So, it can be inferred that the southern boundary of the arid zone in China during the Oligocene was probably near the Lanzhou Basin. The Asian monsoon-arid environment system began to develop during the Oligocene, but the exact position of the arid zone boundary is uncertain in the Oligocene. There were two sizes of mammals in the Nanpoping fauna from the Oligocene strata of the Lanzhou Basin: the small ones were grassland rodents such as Ctenodactylidae; the large ones were forest dwellers such as Paraceratherium and Paraentelodon. Of the 23 species of mammals identified in the Nanpoping fauna, ~ 40% were The Nanpoping fauna was compared with the contemporaneous Dingdanggou fauna in the Danghe area to the north and Jiaozigou fauna in the Linxia area to the south. The Nanpoping fauna contains elements of both other faunas and thus representative a transitional assemblage between forest and grassland. So, it can be inferred that the southern boundary of the arid zone in China during the Oligo cene was probably near the Lanzhou Basin.