Self-assembled silicon nanotubes (SiNTs) with one-dimensional structure have been synthesized from silicon monoxide powder under supercritical hydrothermal conditions with temperature of 470℃, pressure of 6.8 MPa. The silicon nanotubes were identified by transmission electron microscopy, high-resolution transmission electron microscopy, selected area electron diffraction analysis and energy dispersive X-ray spectrum. The results show that the silicon nanotubes are multiwall silicon nanotubes and have close caps at the tips. The average diameter is 13 nm and the diameter of inner pore is smaller than 5 nm in general. The length of SiNTs is up to several microns. The structures of the silicon nanotubes are hollow inner pore, crystalline silicon wall layers with a 0.31 nm interplanar spacing and 2-3 nm amorphous silica outer layers. Some defects in the closed cap of silicon nanotubes are more than that of tubular body. The defects may play an important role in forming the semicircular cap and causing the shift of closed cap from the growth axis of silicon nanotubes. Pure crystalline silicon nanotubes survive after etching the silicon nanotubes with 5% HF acid for enough time which means that the self-assembled silicon nanotubes are stable. The research results show that the stability of self-assembled silicon nanotubes is closely relative to the hydrothermal growth process of silicon nanotubes. The possible theoretical reason for the stable growth of silicon nanotubes from silicon monoxide under supercritically hydrothermal conditions was proposed.