Present paper is an outcome of a four-year floristic survey work carried out in Upper Siang district of eastern Himalayan region in India which is also recognized as one of the 39 known ``Biodiversity Hot-Spots`` of the world. We present here with a vegetation analysis of this area along with a checklist of 1,003 taxa belonging to 110 families, 529 genera, 994 species, 1 subspecies and 8 varieties of the angiosperms. Besides, one new species, 3 new records to India and 82 endemic species were also recorded. The diversity of herbs and shrubs were found higher than the tree species. Orchidaceae was found the most dominant family followed by Poaceae and Fabaceae. Many threatened plants have also been reported from the area. However, rapid urbanization and Jhum cultivation are imposing direct threat to the natural vegetation. We suggest some conservation measures that could help protecting this natural heritage.