Morphological evolution of Si nanowires (Si-NWs) grown on Si (001) substrates is explored. The Si-NWs are fabricated by nanoscale Au-Si island-catalyzed rapid thermal chemical vapor deposition. The Au-Si islands (10-50 nm in dia.) are formed by deposition of Au thin film (1.2-3.0 nm) at room temperature and followed by annealing at 700oC. The Si-NWs are grown by exposure them to a mixture of gasses of SiH4 and H2. We found a critical thickness of the Au film for Si-NW nucleation at a given growth condition. Also, we observed variation in the growth rate and the dimension of the NWs depending on the growth pressure and temperature. The resulting NWs are ~30-100nm in diameter and ~0.4-5.0μm in length. Most of the NWs were aligned along the <111> direction. The morphological and dimensional evolution of the Si-NWs is discussed in terms of kinetics (atomic diffusion mechanism) and energetics (surface and interface energies).