The lasing properties in ZnO thin films grown by laser molecular beam epitaxy on sapphire substrates have been studies. The excitation dependence of the optical gain spectra has been obtained at room temperature using a variable stripe length method. At moderate excitation intensities, the optical gain responsible for lasing is shown as a result of an exciton-exciton scattering process. At higher excitation intensities, the exciton-hole plasma processes become more dominant and exhibit the highest gain reaching value 571 cm at an excitation density of 280 kW/cm. An optical loss in ZnO thin film at room temperature was found to be a low value of 68 cm. A lasing threshold of 85 kW/cm has been observed at room temperature. © 2003 Elsevier B.V. All rights reserved.