The precursory processes detected from unambiguous and repeatable instrumental observations that precede an earthquake remain elusive despite the multiple types of pre-earthquake signals gained from observations of geo-electricity, geomagnetism, and electromagnetism. Recently, much attention has been paid to associate abnormal behaviors of TEC (total electron content) in ionosphere, with seismic forcing. In this paper, we examined ionospheric TEC variations 1 - 2 weeks preceding 20 moderate to great earthquakes (M = 5 - 8) in the Tibetan Plateau and its neighboring regions between 1999 to 2008, with the help of a nationwide continuously-tracking GPS network. The temporal and spatial TEC variations over the specific seismogenic zones were calculated, and the causal linkage between the identified TEC anomalies and these earthquakes was examined. We find that most of the earthquakes showed significant abnormalities with similar characteristics. The anomalies, either upper anomalies (85%, 17/20) or lower anomalies (65%, 13/20) occurred in the ionosphere with dimensions of 30¢X in latitude and 30¢X in longitude above the epicenters. It is noted that the ionospheric anomalies were more dependent on focal depths of earthquakes than their magnitudes. Our results suggest that these anomalies of TEC may be possible seismo-ionospheric signatures for the earthquakes in Tibet and its margins.