Core–shell structured SiC@SiO₂ nanowires and Si@SiO₂ nanowires were prepared on the surface of carbon/carbon (C/C) composites by a thermal evaporation method using SiO powders as the silicon source and Ni(NO₃)₂ as the catalyst. The average diameters of SiC@SiO₂ nanowires and Si@SiO₂ nanowires are about 145 nm, and the core–shell diameter ratios are about 0.41 and 0.53, respectively. The SiO₂ shells of such two nanowires resulted from the reaction between SiO and CO and the reaction of SiO itself, respectively, based on the model analysis. The growth of these two nanowires conformed to the vapor–liquid–solid (VLS) mode. In this mode, CO played an important role in the growth of nanowires. There existed a critical partial pressure of CO (p_C) determining the microstructure evolution of nanowires into whether SiC@SiO₂ or Si@SiO₂. The value of p_C was calculated to be 4.01×10⁻¹⁵ Pa from the thermodynamic computation. Once the CO partial pressure in the system was greater than the p_C, SiO tended to react with CO, causing the formation of SiC@SiO₂ nanowires. However, the decomposition of SiO played a predominant role and the products mainly consisted of Si@SiO₂ nanowires. This work may be helpful for the regulation of the growth process and the understanding of the growth mechanism of silicon-based nanowires.