Exploration of one-dimensional semiconductor nanostruc-tures has led to great progress in the areas of optoelectronics in the past few years. [1] Nanolasers, [2±4] waveguides, [5] frequency converters (second or third harmonic generators), [6] photoconductive optical switches, [7] and sensors [8,9] have been developed based on oxide nanowires. The single crystalline nature of these nanowires makes them ideal candidates for probing size-dependent and dimensionality-controlled physical phenomena. In particular, the transverse nanoscale and longitudinal microscale dimensions (i.e., large aspect ratio) as well as well-defined faceting nature of such nanostructures enable the observation of unique optical confinement and mi-crocavity effects. [2±4] Previously, hexagonal cylindrical ZnO nanowires have been examined as a laser gain medium. These nanocylinders indeed can serve as miniaturized Fabry± Perot optical cavities in the ultraviolet (UV) region with high gain and quality factor. [10] Based on classical waveguide theory , different transverse optical modes can be sustained within waveguides of different cross-sections. [11] It is thus fundamentally interesting to examine the optical cavity effects within nanowires with cross-sections other than hexagonal. Herein, we examine the lasing phenomenon from ZnO nanoribbons with pseudo-rectangular cross-section. Cavity-length dependent optical mode analysis reveals different cavity effects from those of hexagonal nanocylinders. ZnO, being environmentally benign and having a large bandgap (3.37 eV) and exciton binding energy (60 meV), has been considered as a promising candidate for UV light-emitting diodes and laser diodes. It has also displayed an astonishing series of nanostructures with different morphologies. Among many others, the hexagonal nanocylinders, [2±4] nano-ribbons, [5,13] tetrapods, [12] and comb-like nanowire arrays [14] are highly interesting for their fundamental significance in revealing microcavity effects as well as near-field optical coupling phenomena. The ZnO nanoribbons in this work were synthesized using two methodologies. One is a carbon thermal reduction process at 900 C. [12] The other is the high temperature (1350 C) approach developed by Wang and co-workers. [13] The two methods involve different growth mechanisms. The low temperature process utilized Au as the growth initiator. The observation of Au nanoparticles on the nanoribbon tips in the transmission electron microscope (TEM, Figure 1B) suggests that this is a vapor±liquid±solid (VLS) growth process. In contrast , the high-temperature approach is a vapor±solid (VS) growth process without any foreign metal initiation. Regardless of these different growth mechanisms, the growth directions and side facets of the produced nanoribbons are identical for the two sets of the samples. The length of the …