This article describes for the first time the direct observation of the nucleation-and growth process of US nanowires in a typical vapor-solid synthetic route. Thermal evaporating of US nanosized powders at 1173 K for various time durations corresponds to different growth stages and leads to varied product morphologies. Initially, US appeared as amorphous spherical particles, followed by nucleation of nanorods from cusps on the particle surface. Subsequently, nanorods developed to nanowires with matrix particles being consumed. Strategies could be,optimized to grow US nanowires of varied diameters and lengths for a variety of applications.