We used 50-MHz ground-penetrating radar to detect the boundary of permafrost in Xidatan region, the northern limit of the marginal and sporadic permafrost area on the Tibetan Plateau, China. We interpreted the lower limit of permafrost from nine radar profiles to determine the boundary between the permafrost and non-permafrost region. We also sketched the locations of permafrost table along the nine profiles and acquired the lower alpine limit of permafrost in this area. Based on the lower alpine limit of permafrost determined in this field survey, a new permafrost distribution map was drawn with the help of GIS analyses. Comparing with the research work in 1975, permafrost in Xidatan is reduced 12% in area and the lower alpine limit has increased by 25 m. The situation of global warming, apparent climatic warming recorded at two stations near Xidatan, could be responsible for permafrost degradation in this area.