The recrystallization of silver sulfide Ag2S nanoparticles has been studied and the range of the thermal stability of the nanoparticle sizes has been determined. Nanopowders Ag2S with particle sizes of 45– 50 nm were obtained by chemical deposition from aqueous solutions. To study the thermal stability of the Ag2S nanoparticle sizes, the nanocrystalline powders have been annealed in a vacuum of 0.01 Pa on heating from room temperature to 493 K and in argon at 623 K. Annealing up to a temperature of 453 K leads to insignificant nanoparticle growth and annealing of microstrains, which allows one to consider this temperature range as the region of thermal stability of the silver sulfide nanostate. The temperature range from 450 to 900 K, in which the particle size increases by a factor of 3–6, corresponds to the temperature of collective recrystallization of the silver sulfide nanopowder.