The effect of surface structures of annealed WO3 thin films on heteroepitaxial growth of molecular beam epitaxy (MBE) deposited silver nanoparticles (Ag NPs) has been investigated. Hexagonal WO3 (h-WO3) and monoclinic WO3 (m-WO3) thin films were used in (1x1) stoichiometric forms. The structure, the density, the particle size and the morphology of Ag NPs on WO3 films were determined by transmission electron microscope (TEM) and selected area electron diffraction (SAEC). Ag NPs on the m-WO3 surface created the (111)Ag//(001)m-WO3 interface without preferred orientations while the epitaxial relationships between Ag NPs and the h-WO3 films were formed as (111)Ag//(0001)h-WO3 and Ag[01-1]//[21-30]h-WO3. It was observed that the particle density, the mean size and the size dispersion of Ag NPs were controlled by either atom diffusion or by particle migration on the substrates. The maximum cluster density method and the Lifethenz theory of Van der Waals energy were used to investigate the Ag NP morphology and Ag//WO3 interface parameters at saturation particle density of NPs.