Silver-gold nanocages have attracted considerable research interest recently due to their excellent performance in the fields of biomedicine and photocatalysis. These applications oftentimes manipulate at elevated temperatures and therefore impose demands on the thermal stability of the cage structures. To better understand this subject, in this work, we systematically evaluated the thermal stability of two nanocages with different wall thicknesses of 3.8 nm and 13 nm, both in the solution-phase (diethylene glycol) and solid-phase (in-situ STEM). The results revealed that the nanocages with thicker walls exhibited better thermal stabilities in both phases. By monitoring and analyzing the morphology changes of the nanocages, we determined that the nanocages with thin and thick walls undergo deformation processes differently. Nevertheless, they both deformed into more thermodynamically stable structures eventually. The plasmonic properties of the nanocages were also examined.