We report a new two-dimensional structure, ZnO single crystal flakes, synthesized using a simple combustion CVD process. The flakes grew upward from the Si substrate. The as-grown ZnO flakes were less than 200 nm thick and 10–20 µm wide. Unlike most 1-D ZnO structures with [0001] growth direction, ZnO flakes grow along <100> directions. A gas sensor based on a 200 nm thick ZnO flake exhibited fast response–recovery and high sensitivity to ethanol vapor. These as-synthesized single crystal ZnO flakes could have important technological applications.