The strikes of rifts in western Tibet show NNW-SSE trending, whereas the rifts in eastern Tibet strike in the NNE-SSW direction. The difference in the rift strike orientation suggests that the formation mechanisms of these rifts are different in southern Tibet, which can be most reliably inferred from seismic structure beneath these rifts. In this work we study 3-D P wave velocity structure and radial anisotropy using a large number of travel time data recorded by the ANTILOPE and Hi-CLIMB portable seismic arrays deployed in central Tibet. Our results show that to the west of the Pumuqu Xianza rift, a low-velocity zone with a positive radial anisotropy exists beneath the Lopu Kangri rift (similar to 85 degrees E), which may reflect melt-filled cracks. A high-velocity zone with a negative radial anisotropy is revealed beneath the Pumuqu Xianza rift (similar to 88 degrees E), which reflects lithospheric downwelling. The different upper mantle structures may result in the patterns of rift strike orientation in central Tibet.