Well-aligned ZnO nanowires were successfully synthesized on a silicon substrate at the low temperature of 550 °C by catalyst-free vapour phase deposition. The ZnO nanowires had diameters in the range of 70-100 nm and lengths over several tens of micrometres. The synthesized ZnO nanowires, which had a single-crystalline wurtzite structure, showed a uniform morphology and faceted planes at the tips of the nanowires. The photoluminescence of the ZnO nanowires showed a strong UV band at 3.28 eV and a broad green band at 2.29 and at 2.53 eV at room temperature. A detailed discussion regarding the growth behaviour and the growth mechanism of the ZnO nanowires on the silicon substrate is presented in this work.