The Qiongduojiang graben, which is located in the central part of the Cuona- Oiga rift in the eastern end of the near NS- trending rift in South Tibet, is a half- graben faulting basin controlled mainly by the high- angle normal fault in the western margin of the graben. Detailed geological and geomorphic survey shows that the basin is filled with several beds of fluvio- lacustrine, moraine and outwash sediments since the Late Cenozoic. The fluvio- lacustrine sequence is dominantly clay and silt stone at bottom, gravels at the upper part with the diameter increasing gradually, and early Pleistocene alluvial gravel at top. Moraine is mainly developed in the piedmont zone of the central graben, forming broad and gentle glacial terraces and lateral moraine dams. The dating results of the sediments indicate that there are two sets of fluvio- lacustrine deposited before 5 Ma and Quaternary respectively and moraine and outwash sediments formed since the middle Pleistocene. Comprehensive analysis of geology, geomorphology, sedimentary and structure features suggest that the old fluviolacustrine sediments were related to intensive vertical faulting of the main basin boundary after initial rifting of the basin, while late fluvio- lacustrine sediments formed during the process of shrinking of the basin, and were probably related to the glacial lake since the middle Pleistocene. Because the Qiongduojiang graben was controlled by the main boundary normal fault of the western margin, the early fluvio- lacustrine deposition should be later than initial rifting time. Thus, further integration with the existing age data suggests that the initial rifting age of the South Tibet rifts should be older than 5- 10Ma and younger than ca.15Ma.