Two-dimensional (2D) materials have attracted great attention due to their unique physical and chemical properties derived from the dimensionality effect and modulation in their band structure. Herein, an important industrial catalytic material, gamma alumina (gamma-Al2O3) with well-defined two-dimensional nanosheet structure was fabricated by a designed crown ether induced assembly approach. The obtained gamma-Al2O3 nanosheets with a thickness of similar to 4 nm possess high surface area (351 m(2)/g) and 16% pentacoordinate Al3+ sites which are the main catalytic active sites for ethanol dehydration to ethylene, compared to almost no pentacoordinate Al3+ sites for the commercial gamma-Al2O3 nanoparticles. Furtherly, systematical investigation reveals that the crown ether plays the key role in the assembly of nanosheet structure. As a catalyst for ethanol dehydration to ethylene, the obtained alumina nanosheets with easily-accessible surface and more active sites, exhibit a superior catalytic performance to the commercial alumina with a much higher yield at large space velocity.