Highly pure, ultralong, and uniform-sized semiconductor nanowires in bulk quantity were synthesized by thermal evaporation or laser ablation of semiconductor powders mixed with oxides. Transmission electron microscopy study shows that decomposition of semiconductor suboxides and defect structures play important roles in enhancing the formation and growth of high-quality nanowires. A new growth mechanism is proposed on the basis of microstructure and different morphologies of the nanowires observed.