In this work, the thermal stability of annealed Ag/Nb multilayers was investigated as the individual layer thickness (h) varies from 1 nm to 100 nm. The lamellar structures remain unchanged after annealing at 500 °C for 1 h, and the hardness at all scales remains undiminished after annealing. The results show that two low energy interfaces are responsible for the superior thermal stabilities. The coherent interface is formed when h < 20 nm, and the amorphous interface is formed when h ≥ 20 nm, respectively. The evolution of interfacial structures and the process of solid state amorphization are discussed accordingly.