The effects of silica addition on the thermal stability of flame-synthesized ${gamma}-Al_2O_3$ nanoparticles are investigated in this study. Based on the electron microscope images of pure alumina, nanoparticles with diameters ranging from 20 nm to 30 nm were synthesized. No significant changes were observed after heat treatment at $900^{circ}C$, $1100^{circ}C$, and $1180^{circ}C$. After heat treatment at $1260^{circ}C$, sintered particles larger than 100 nm were observed. All images of the nanoparticles doped with silica, including those taken after heat treatment at $1260^{circ}C$, showed similar particle sizes regardless of treatment temperature. Based on the X-ray diffraction patterns and specific surface areas, the onset temperature of the phase transition to ${alpha}-Al_2O_3$ for the flame-synthesized pure alumina was near $1180^{circ}C$. The addition of silica greatly increased the thermal stability of the synthesized ${gamma}-Al_2O_3$ nanoparticles.