The Cenozoic volcanic rocks at the center of Qiangtang are located in the area where the curst of Qianghai-Tibetan Plateau thins from south to north with an unusually concomitant increase of Paisson′s ratio. The ages of the volcanic rocks range from 44.1±1.0Ma to 32.6± 0.8Ma, older than other rocks in northern Tibet. Petrological and geochemical studies suggest that the uniform elevation of northern Tibetan Plateau is closely related to the decoupling of the lithospheric mantle, crustal extension and thinning and volcanism. Therefore, the authors infer that the beginning of the main uplift of the northern plateau occurred at about 40 Ma ago.