This paper uses the travel times of 78364 Pn waves and seismic tomography to invert for the Pn velocity structure at the top of upper mantle in Mongolia and neighboring regions. The result indicates that the Pn velocity is 8. 08 km/s on average and varies laterally between -0.39 km/s and +0.30 km/s. The high velocity anomalies are mainly distributed beneath the Altay mountains, ]unggar basin, and Tarim basin; in the Altay mountain region the largest Pn anomaly reaches +0.30 km/s. The regions of lower velocity include mainly Lake Baikal, North China Plain, and Bohai Bay~ to the southwest of Lake Baikal the strong low velocity anomaly reaches -0.39 km/s, which is probably related to the Cenozoic volcanic activity in the area.