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Nanoneedles are novel silicon nanostructures. With the use of silicon monoxide as the starting material, a small quantity of deionized water as the reaction medium, and argon as the protection gas, silicon nanoneedles were successfully synthesized. The growth conditions were controlled at 490°C with a pressure of 3.2-3.5 MPa. High-resolution transmission electron microscopy (HRTEM), selected-area electron diffraction (SAED) and electron energy-dispersive spectroscopy (EDS) revealed that the as-grown nanoneedles had pyramidal tips with the diamond structure. The chemical composition at the pyramidal tip was silicon with a small amount of oxygen. Compared with single-crystal silicon, the Raman spectra of silicon nanoneedles were found to be downshifted because of their small size.