Nanoribbons and flat nanosheets of Ga2O3 have been synthesized by evaporating GaN at high temperature with the presence of oxygen. The as-synthesized nanoribbons and nanosheets are pure, structurally uniform, single crystalline, and free from dislocations. The nanoribbons and the nanosheets all have monoclinic beta-Ga2O3 structure. The flat top and bottom surfaces for both nanoribbons and nanosheets are ( 100), tile side surfaces are +/-(010) and +/-(10 (1) over bar) for nanoribbons and +/-(010), +/-(10 (1) over bar) and +/-(21 (2) over bar) for nanosheets. The axis direction of nanoribbon growth is along either [001] or [010].