Samples containing Ag nanoparticles (NPs) in a ZnO matrix were produced using a sol-gel method, with varying the annealing temperature and the thickness of the ZnO cupping layer. The dependence of the optical and structural properties of the samples on the silver diffusion was studied. As a result of annealing at temperatures of 570-650 °С, a long-wavelength shift of the plasmon absorption peak of NPs was observed due to an increase in their average size, as well as an increase in the intensity of the absorption peak due to a narrower size distribution of NPs. The transformation of the nanoparticle shape and size during annealing was shown to result from the diffusion of silver, whereas the ZnO cupping layer and its thickness was proved to control these processes.