By varying the final heating temperature in the range 1050°C - 1300°C, cubic silicon carbide (β-SiC) and/or trigonal silicon nitride (α-Si₃N₄) nanowires (NWs) were prepared by direct thermal treatment under nitrogen, of commercial silicon powder and graphite. Long and highly curved β-SiC NWs were preferentially grown below 1200°C, while straight and short α-Si₃N₄ NWs were formed above 1300°C. Between these two temperatures, a mixture of both nanowires was obtained. The structure and chemical composition of these nanostructures have been investigated by SEM, HRTEM, EDX and EELS.