A new method to reduce the dislocation density in a GaN film grown on sapphire substrate by metalorganic chemical vapor deposition (MOCVD) is reported. In this new method, SiH4 and NH3 gases are simultaneously introduced at a low temperature with a certain time before the growth of an initial low-temperature GaN buffer layer. By transmission electron microscope (TEM), the density of threading dislocation originating from the interface between low-temperature buffer layer and high-temperature GaN layer decreases to be almost invisible from 7x10(8)/cm(2) in the conventional MOCVD growth technology for GaN film. Atomic force microscopy indicates that introducing SiH4 and NH3 gases at a low temperature changes surface morphology, which probably enhances the lateral growth and then decreases the dislocation density. This method could be used for fabrication of long-lifetime GaN-based laser instead of epitaxial lateral overgrowth.