We have observed ultraviolet laser emission at 3.2eV from optically pumped ZnO nanocrystal thin films at room temperature. ZnO films were epitaxially grown on sapphire substrates by laser molecular beam epitaxy. Closely packed and hexagonally shaped nanocrystals and well-defined grain boundaries were naturally formed. The grain boundaries between nanocrystals serve not only as barriers for confinement of excitons, but also as mirrors to form a Fabry-Perot cavity.