Si nanowires are synthesized at different temperature, 970 degreesC and 1080 degreesC, on Si substrate by gas condensation of pure SiO vapor without any metal catalysts. Scanning electron microscopy and transmission electron microscopy show the morphology of the synthesized nanostructures is significantly altered by temperature: in specimens prepared under 970 degreesC, dispersed nanoparticles, nanoparticle assemblies and smooth nanowires are discovered at different deposition densities; in specimens prepared under 1080 degreesC, only short nanowires growing directly from the substrate can be seen. The observation indicates there are different growth processes for the two kinds of morphologies. Their growth mechanisms are discussed. A model based on sintering, diffusion and clustering effects is suggested for the low-temperature process. (C) 2004 Elsevier B.V. All rights reserved.