The paper reports morphology and structure transitions of silicon carbide (SiC) nanowires during high temperature annealing; the as-prepared nanowires are in the form of SiC core and SiO2 shell. The transition temperature is about 1200 degrees C, 600 degrees C lower than that of SiC microfibers, and it starts with the formation of junctions of individual nanowires. The junctions grow into webs while the crystalline SiC cores of the nanowires oxidize. The growth and the oxidation eventually lead to the formation of an oxide film, when the transition completes. The thermal stability and the transition mechanisms of SiC nanowires are critical to their applications in high temperature environments.