Amorphous silicon nanowires (NWs), 3-50 nm thick and up to 4 mu m long, were grown by room-temperature continuous wave laser ablation of Si in high-pressure (0.1-0.9 MPa) Ar gas without the addition of any catalysts. The diameter and length of the NWs increased as the pressures of the ambient Ar increased. Sphere-like Si particles with diameters of 4-110 nm were observed at the tips of grown NWs and their diameters exhibited a strong correlation with the NW diameters. We propose a stress-driven self-catalytic vapor-liquid-solid mechanism to explain the growth of the NWs.