A three-dimensional P wave velocity model of the crust and upper mantle down to 400-km depth beneath eastern Tibet is obtained using many temporary seismic stations of the ASCENT project and the Namche Barwa Broadband Seismic Network. We collected 16,508 arrival times of P, Pn and Pg phases from 573 local and regional earthquakes and 7,450 P wave arrivals from seismograms of 435 teleseismic events. Our high-quality data set enables us to reconstruct the 3-D velocity structure under eastern Tibet in more detail than the previous studies. In the shallow depth, our results show that the low velocity zones are not interconnected well (no wide-spread low velocity zones), which may reflect the complex pattern of material flowing in the study region. Below the Moho, we find that the Indian lithospheric mantle underthrusts sub-horizontally under eastern Tibet, and the extent of the northward advancing Indian lithosphere decreases from west to east. In the north, the Asian lithospheric mantle is detected under the vicinity of the Qaidam Basin. Between the Indian and Asian lithospheric mantles, there is an obvious low-velocity anomaly which may reflect an upwelling mantle diapir.