Silicon nanowires have been grown by the thermal decomposition of silane via the vapour-liquid-solid (VLS)mechanism. Three different stages of VLS growth (eutectic alloy formation, crystal nucleation and unidirectional growth)were studied separately using a scanning electron microscope and a high-resolution transmission electron microscope.Very short silicon nanowires prepared under particular conditions provide direct evidence of the VLS mechanism on a nanometre scale. Our results will be very helpful for the controllable synthesis of Si nanowires.