Ab　initio　energetic　calculations　based　on　the　density　functional　theory　(DFT)　and　projector　augmented　wave　(PAW)　pseudo-potentials　method　were　performanced　to　determine　the　crystal　structural　parameters　and　phase　transition　data　of　the　polymorphic　rare-earth　sesquioxides　Ln(2)O(3)　(where　Ln　=　La　-Lu,　Y,　and　Sc)　with　A-type　(hexagonal)　and　B-type　(monoclinic)　con.　gurations　at　ground　state.　The　calculated　results　agree　well　with　the　limited　experimental　data　and　the　critically　assessed　results.　A　set　of　systematic　and　self-consistent　crystal　structural　parameters,　energies　and　pressures　of　the　phase　transition　were　established　for　the　whole　series　of　the　A-and　B-type　rare-earth　sesquioxides　Ln(2)O(3).　With　the　increase　of　the　atomic　number,　the　ionic　radii　of　rare-earth　elements　Ln　and　the　volumes　of　the　sesquioxides　Ln(2)O(3)　reflect　the　so-called　＂lanthanide　contraction＇＇.　With　the　increase　of　the　Ln(3+)　-cation　radius,　the　bulk　modulus　of　Ln(2)O(3)　decreases　and　the　polymorphic　structures　show　a　degenerative　tendency.　(C)　2007　Elsevier　Inc.　All　rights　reserved.