Abstract The structure of GaSe thin films condensed on glass substrates by vacuum evaporation at a residual pressure of 10 -6 torr has been investigated by X-ray diffraction and transmission electron microscopy techniques. A best fit to the observed diffraction data gave the hexagonal lattice parameters a = 3.78 A ̊ and c = 15.94 A ̊ . The films consisted of plate-like crystals with their c axis oriented normal to the plane of the film and a axis oriented at random. The influence of substrate temperature and deposition rate on film structure was investigated. Varying the substrate temperature from room temperature to 220°C, it was found that the lateral crystallite size increased from 250 Å to 5000 Å, and at 220 °C large single crystals were formed in the film. Increasing the deposition rate from 5 Å/sec to 20 Å/sec caused a 50 percent decrease in the crystallite size. Post deposition vacuum annealing at 230 °C considerably improved the crystallinity of the films and large single crystals were formed due to the removal of defects.