Twinned SiC nanowires were prepared on a silicon wafer by a catalyst-free thermal chemical vapour deposition (CVD) method at 1500 °C in a flowing Ar atmosphere. X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), energy dispersive spectroscopy (EDS), transmission electron microscopy (TEM), and high-resolution transmission electron microscopy (HRTEM) were used to characterise the phase composition, morphology, and microstructure of the as-received nanowires. The as-synthesised twinned nanowires were up to several hundred microns long and had relatively homogeneous diameters in the range 20 to 100 nm. The growth process of the twinned SiC nanowires was dominated by a vapour–solid (VS) mechanism. Based on the competition of stacking fault energy and electrostatic energy, a growth model has been proposed to understand the phenomenon of twinning.